Have a Happy New Year from
"GARY'S NAUTICAL INFORMATION"
Monday, December 31, 2007
COMPOSITE SAILING
Composite sailing.-When the great circle would carry a vessel to a higher latitude than desired, a modification of great-circle sailing, called composite sailing, may be used to good advantage. The composite track consists of a great circle from the point of departure and tangent to the limiting parallel, a course line along the parallel, and a great circle tangent to the limiting parallel and through the destination.
Solution of composite sailing problems is most easily made by means of a greatcircle chart. Lines from the point of departure and the destination are drawn tangent to the limiting parallel. The coordinates of various selected points along the composite track are then measured and transferred to a Mercator chart. Composite sailing problems can also be solved by computation.
Solution of composite sailing problems is most easily made by means of a greatcircle chart. Lines from the point of departure and the destination are drawn tangent to the limiting parallel. The coordinates of various selected points along the composite track are then measured and transferred to a Mercator chart. Composite sailing problems can also be solved by computation.
EXPLAINATION OF PARALLEL SAILING
Parallel sailing was the navigators inability to determine his longitude. Not a mathematical solution in the sense that the other sailings are, it involved converting the distance sailed along a parallel (departure), as determined by dead reckoning, into longitude. It is the interconversion of departure and difference of longitude when a vessel is proceeding due East or due West. This was a common occurrence when the sailings were first employed several hundred years ago.
Without knowledge of his longitude, the navigator of old found it necessary on an ocean crossing to sail northward or southward to the latitude of his destination, and then to follow that parallel of latitude until the destination was reached, even though this might take him far out of his way. Because of this practice, parallel sailing was an important part of the navigator's store of knowledge. The method was a crude one, and the time of landfall was often in error by a matter of days, and, in some cases even weeks.
The formulas for Parallel Sailings are:
DLo = p sec L
P = DLo cos L
Without knowledge of his longitude, the navigator of old found it necessary on an ocean crossing to sail northward or southward to the latitude of his destination, and then to follow that parallel of latitude until the destination was reached, even though this might take him far out of his way. Because of this practice, parallel sailing was an important part of the navigator's store of knowledge. The method was a crude one, and the time of landfall was often in error by a matter of days, and, in some cases even weeks.
The formulas for Parallel Sailings are:
DLo = p sec L
P = DLo cos L
Nautical Info and Trivia
1. The date is the same all over the world at ?
Answer - 1200 GMT.
2. The reference point for determination of GMT is the passage of the mean sun over what line?
Answer - 180 degree's Longitude.
3. A mean sun is ?
Answer - A fictitious sun conceived to move eastward along the celestial equator.
4. 1 fathom is equal to ?
Answer - 6 feet.
5. 1 cable is equal to ?
Answer - 720 feet, also 0.1 nautical mile (British).
6. What is a Fidley Deck ?
Answer - A partially raised deck over the engine.
7. Charley Noble - A British merchant service captain, Charles Noble, is said to be responsible for the origin, for the galley smokestack. It seems that Captain Noble, discovering that the stack of his ship's galley was made of copper, ordered that it be kept bright. The ship's crew then started referring to the stack as the "Charley Noble."
8. Holystone - often used to scrub the decks of ships. Sailors had to kneel as if in prayer when scrubbing the decks. Holystone was often called so because it is full of holes.
9. Jack Tar a slang term for a Sailor, The term "Jack tar" was used by the 1780s early sailors wore overalls and broad-brimmed hats made of tar-impregnated fabric called tarpaulin cloth. The hats, and the sailors who wore them, were called tarpaulins, which may have been shortened to tars.
10. Whistling for Wind: Based on a very old tradition that whistling at sea will cause a wind to rise.
Sunday, December 30, 2007
SHIPS BELL
Bells have a centuries long tradition of varied use in the navies and merchant fleets of the world. They have been used for signaling, keeping time, and providing alarm. Their functional and ceremonial uses have made them a symbol of considerable significance to the United States Navy and merchant ships.
Bells for warning and alarms
The sounding of a ship's bell found a natural application as a warning signal to other vessels in poor visibility and fog. In 1676 one Henry Teonage serving as a chaplain in the British Mediterranean Fleet recorded , "so great a fog that we were fain to ring our bells, beat drums, and fire muskets often to keep us from falling foul one upon another". Ringing a ship's bell in fog became customary. In 1858, British Naval Regulations made it mandatory in that function. Today, maritime law requires all ships to carry an efficient bell.
Bells for warning and alarms
The sounding of a ship's bell found a natural application as a warning signal to other vessels in poor visibility and fog. In 1676 one Henry Teonage serving as a chaplain in the British Mediterranean Fleet recorded , "so great a fog that we were fain to ring our bells, beat drums, and fire muskets often to keep us from falling foul one upon another". Ringing a ship's bell in fog became customary. In 1858, British Naval Regulations made it mandatory in that function. Today, maritime law requires all ships to carry an efficient bell.
American ships of the Revolutionary War period and our early national years adopted many of the practices and traditions of the British Royal Navy, including the use of bells. In 1798, Paul Revere cast a bell weighing 242 pounds for the frigate Constitution, also known today by its nickname "Old Ironsides".
It is of interesting to note that the use of a ship's bell contributed to the richest single prize captured by the American Navy during the War of Independence. While a Continental Squadron under Commodore Whipple lay-to, wrapped in Newfoundland fog in a July morning in 1779, the sound of ships' bells and an occasional signal gun could be heard a short distance off. When the fog lifted the Americans discovered that they had fallen in with the richly laden enemy Jamaica Fleet. Ten ships were captured as prizes, which together with their cargo were valued at more than a million dollars.
Bells for timekeeping
Before the advent of the chronometer time at sea was measured by the trickle of sand through a half - hour glass. One of the ship's boys had the duty of watching the glass and turning it when the sand had run out. When he turned the glass, he struck the bell as a signal that he had performed this vital function. From this ringing of the bell as the glass was turned evolved the tradition of striking the bell once at the end of the first half hour of a four hour watch, twice after the first hour, etc., until eight bells marked the end of the four hour watch. The process was repeated for the succeeding watches. Bells for alarms
The bell is an essential link in a ship's fire alarm system. In the event of a fire, the bell is rung rapidly for at least five seconds, followed by one, two or three rings to indicate the location of a fire - forward, amidships, or aft respectively.
Before the advent of the chronometer time at sea was measured by the trickle of sand through a half - hour glass. One of the ship's boys had the duty of watching the glass and turning it when the sand had run out. When he turned the glass, he struck the bell as a signal that he had performed this vital function. From this ringing of the bell as the glass was turned evolved the tradition of striking the bell once at the end of the first half hour of a four hour watch, twice after the first hour, etc., until eight bells marked the end of the four hour watch. The process was repeated for the succeeding watches. Bells for alarms
The bell is an essential link in a ship's fire alarm system. In the event of a fire, the bell is rung rapidly for at least five seconds, followed by one, two or three rings to indicate the location of a fire - forward, amidships, or aft respectively.
Maintenance and upkeep
Traditionally, the bell is maintained by the ship's cook, while the ship's whistle is maintained by the ship's bugler. In actual practice, the bell is maintained by a person of the ship's division charged with the upkeep of that part of the ship where the bell is located. In such a case a deck seaman or quartermaster striker or signalman striker may have the bell shining duty.
Today's role for bells
In addition to continuing its role as a timepiece and alarm, the bell serves a ceremonial and memorial function. Bells remain a powerful and tangible reminder of the history, heritage, and accomplishments of the naval service and merchant fleet.
Traditionally, the bell is maintained by the ship's cook, while the ship's whistle is maintained by the ship's bugler. In actual practice, the bell is maintained by a person of the ship's division charged with the upkeep of that part of the ship where the bell is located. In such a case a deck seaman or quartermaster striker or signalman striker may have the bell shining duty.
Today's role for bells
In addition to continuing its role as a timepiece and alarm, the bell serves a ceremonial and memorial function. Bells remain a powerful and tangible reminder of the history, heritage, and accomplishments of the naval service and merchant fleet.
Saturday, December 29, 2007
WHAT VIKINGS EAT
The Vikings ate two main meals a day, one of which usually consisted of some kind of meal or porridge. The mainstay of everyday eating was the big kettle of stew (or skause - a Norse word!) containing whatever vegetables and meat were available, and added to day by day. Food was a vital part of domestic life, and the evening meal was the focus for conversation, games, music, and storytelling. In an age where inns were virtually unknown, it was considered a matter of honor to practice hospitality - you never knew when a member of your family would need the hospitality of strangers in their turn. The kind of food we eat today would be a bit fancy for everyday Viking use, but most people who cook authentically understandably want something that looks more inviting and interesting than the grey stew.
Bread was made in great quantity and variety, both flat and risen. It's uncertain if the Vikings had cultivated yeast as we know it, but they certainly made use of wild yeasts, raising agents such as buttermilk and sour milk, and the leftover yeast from brewing. They also used the 'sourdough' method, where a flour and water starter is left for several days to ferment. The most commonly grown cereal crops were oats, rye, and barley, but wheat was also widely used. Flour was also made from nuts (including acorns) or pulses (peas and beans), and even from tree bark.
Dairy
In the Viking age, dairy products formed an important part of the diet. Whole milk was rarely drank (probably because it was too valuable a commodity when made into butter), but buttermilk and whey were popular, as were curds, butter, and cheese. Cheese and butter could be eaten fresh (a rare springtime treat), but were more commonly salted and fermented, to keep over the winter. Milk came not only from cows, but also from sheep, goats, and horses. It was a seasonal product, only available in the spring when the female animals were lactating.
In the Viking age, dairy products formed an important part of the diet. Whole milk was rarely drank (probably because it was too valuable a commodity when made into butter), but buttermilk and whey were popular, as were curds, butter, and cheese. Cheese and butter could be eaten fresh (a rare springtime treat), but were more commonly salted and fermented, to keep over the winter. Milk came not only from cows, but also from sheep, goats, and horses. It was a seasonal product, only available in the spring when the female animals were lactating.
Eggs
These came from chickens, geese, ducks, and all manner of wild birds. Gull's eggs were considered a particular delicacy, and were collected from the clifftops during the spring months.
These came from chickens, geese, ducks, and all manner of wild birds. Gull's eggs were considered a particular delicacy, and were collected from the clifftops during the spring months.
Meat
Meat was available at all levels of society; even poorer folk managed the occasional bit of game or preserved meat. The most common meat animal seems to have been pigs (they breed easily and mature quickly), but sheep, goats, cows and horses were kept both for meat and milk. Horsemeat was forbidden to Christians, and was one of the grounds on which the Church vilified the Vikings. Domestic animals were slaughtered in November (known as Bloodmonth), to avoid having to feed them over the winter, and then preserved by various methods. Game animals included hare, boar, wild birds, squirrel, deer; and, in the far north, reindeer, seal, and polar bear.
Meat was available at all levels of society; even poorer folk managed the occasional bit of game or preserved meat. The most common meat animal seems to have been pigs (they breed easily and mature quickly), but sheep, goats, cows and horses were kept both for meat and milk. Horsemeat was forbidden to Christians, and was one of the grounds on which the Church vilified the Vikings. Domestic animals were slaughtered in November (known as Bloodmonth), to avoid having to feed them over the winter, and then preserved by various methods. Game animals included hare, boar, wild birds, squirrel, deer; and, in the far north, reindeer, seal, and polar bear.
Fish
Freshwater fish such as salmon, trout and eels were widely eaten. In coastal areas there were shellfish and herring, and deep-sea fish such as cod from the rich fishing grounds off the north coast of Norway and Finland. Fish (and meat) were eaten fresh, salted, pickled, smoked
or dried. Herrings were hung out on large frames to dry in the cold wind. Dried herrings were eaten like biscuits, spread with butter. Fish could also be fermented or preserved in whey. If the idea of fermented fish makes you sick, consider what that old favourite Worcester Sauce is made of.Freshwater fish such as salmon, trout and eels were widely eaten. In coastal areas there were shellfish and herring, and deep-sea fish such as cod from the rich fishing grounds off the north coast of Norway and Finland. Fish (and meat) were eaten fresh, salted, pickled, smoked
Vegetables and fruit
Some of the common vegetables we take for granted were unknown to the Vikings; potatoes being an obvious example. Others include orange carrots (Dark Age ones were white.
Some of the common vegetables we take for granted were unknown to the Vikings; potatoes being an obvious example. Others include orange carrots (Dark Age ones were white.
Drink
The universal drink of the Viking period was ale - not as we know it today, but fairly weak, sweetish, cloudy, and often unhopped (a wide variety of herbs were used for flavouring, although the hop trade existed). And then there was buttermilk and whey.
The universal drink of the Viking period was ale - not as we know it today, but fairly weak, sweetish, cloudy, and often unhopped (a wide variety of herbs were used for flavouring, although the hop trade existed). And then there was buttermilk and whey.
VIKINGS
The Vikings were the most powerful people in northwestern Europe for nearly five centuries from about AD 750 to 1100. They were the first Europeans to reach North America, arriving at New Foundland or Vinland around the year 1001. Today, a thousand years after they terrorized coastlines from the eastern Mediterranean to the Atlantic Ocean, the Vikings of Scandinavia are remembered as fearless sailors, merciless raiders, wise traders, and hardy settlers. When thinking of Viking warriors, the picture of a crazy and ruthless barbarians with horned helmets and huge battle-axes in longships, usually comes to mind. That picture doesn't match reality. Horned helmets were rare and not used in battles, only in religious ceremonies. Unfortunately, horned helmets came to represent a savage image of brutal Viking land and sea invaders.
Their longships were fast and sleek, powered by sail or oars, they were ideally suited for raiding because their shallow draught meant that they could travel up estuaries and rivers. The long, narrow ships packed with warriors helped to make the Vikings the dominant power in Europe for nearly five centuries. Two of the most common Viking ships were the knarr and the longboat. The longboat was the biggest ship and was used for raiding. It could be up to 100 feet long and 20 feet wide. The smaller, wider and heavier knarr was used by farmers and merchants to carry a heavy cargo. The knarr had a front and rear deck. Shipbuilders took pride in building beautiful ships and usually decorated the front with a fierce looking carving.
The longboat was built to survive the stormy seas and to sail on shallow rivers. They were also built light enough to be carried over land. When the wind was behind a longboat, the Vikings used large sails. If there wasn't any wind, or it was blowing in the wrong direction, up to 80 warriors could use oars to power the boat.
The longboat was built to survive the stormy seas and to sail on shallow rivers. They were also built light enough to be carried over land. When the wind was behind a longboat, the Vikings used large sails. If there wasn't any wind, or it was blowing in the wrong direction, up to 80 warriors could use oars to power the boat.
Friday, December 28, 2007
METEORITE
A meteorite is a natural object originating in outer space that survives an impact with the Earths surface without being destroyed. While in space it is called a meteoroid. When it enters the atmosphere, air resistance causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.
More generally, a meteorite on the surface of any celestial body is a natural object that has come from elsewhere in space. Meteorites have been found on the Moon and Mars.
Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of mid-2006, there are approximately 1,050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds.
More generally, a meteorite on the surface of any celestial body is a natural object that has come from elsewhere in space. Meteorites have been found on the Moon and Mars.
Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of mid-2006, there are approximately 1,050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds.
Meteorites are always named for the place where they were found, usually a nearby town or geographic feature. In cases where many meteorites were found in one place, the name may be followed by a number or letter (e.g., Allan Hills 84001 or Dimmitt (b)).
Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy
Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy
Great Circle Sailing ( Pub. 229 )
Great Circle Sailing. The great-circle distance between any two points on the assumed spherical surface of the Earth and the initial great-circle course angle may be found by relating the problems to the solution of the celestial triangle. For by entering the tables with latitude of departure as latitude, latitude of destination as declination, and difference of longitude as LHA, the tabular altitude and azimuth angle may be extracted and converted to distance and course.
The tabular azimuth angle becomes the initial great-circle course angle, prefixed N or S for the latitude of departure, and suffixed E or W depending upon the destination being east or west of point of departure.
If all entering arguments are integral degrees, the altitude and azimuth angle are obtained directly from the tables without interpolation. If the latitude of destination is nonintegral, interpolation for the additional minutes of latitude is done as in correcting altitude for any declination increment; if either the latitude of departure or difference of longitude, or both, are nonintegral, the additional interpolation is done graphically.
Since the latitude of destination becomes the declination entry, and all declinations appear on every page, the great-circle solution can always be extracted from the volume which covers the latitude of departure.
Great-circle solutions belong in one of the four following cases:
The tabular azimuth angle becomes the initial great-circle course angle, prefixed N or S for the latitude of departure, and suffixed E or W depending upon the destination being east or west of point of departure.
If all entering arguments are integral degrees, the altitude and azimuth angle are obtained directly from the tables without interpolation. If the latitude of destination is nonintegral, interpolation for the additional minutes of latitude is done as in correcting altitude for any declination increment; if either the latitude of departure or difference of longitude, or both, are nonintegral, the additional interpolation is done graphically.
Since the latitude of destination becomes the declination entry, and all declinations appear on every page, the great-circle solution can always be extracted from the volume which covers the latitude of departure.
Great-circle solutions belong in one of the four following cases:
Case I-Latitudes of departure and destination of same name and initial great-circle distance less than 90°.
Enter the tables with latitude of departure as latitude argument (Same Name), latitude of destination as declination argument, and difference of longitude as local hour angle argument. If the respondents as found on a right-hand page do not lie below the C-S Line, Case III is applicable.
Extract the tabular altitude which subtracted from 90° is the desired great-circle distance. The tabular azimuth angle is the initial great-circle course angle.
Enter the tables with latitude of departure as latitude argument (Same Name), latitude of destination as declination argument, and difference of longitude as local hour angle argument. If the respondents as found on a right-hand page do not lie below the C-S Line, Case III is applicable.
Extract the tabular altitude which subtracted from 90° is the desired great-circle distance. The tabular azimuth angle is the initial great-circle course angle.
Case II-Latitudes of departure and destination of contrary name and great-circle distance less than 90°.
Enter the tables with latitude of departure as latitude argument (Contrary Name) and latitude of destination as declination argument, and with the difference of longitude as local hour angle argument. If the respondents do not lie above the C-S Line on the right-hand page, Case IV is applicable.
Extract the tabular altitude which subtracted from 90° is the desired great-circle distance. The tabular azimuth angle is the initial great-circle course angle.
Enter the tables with latitude of departure as latitude argument (Contrary Name) and latitude of destination as declination argument, and with the difference of longitude as local hour angle argument. If the respondents do not lie above the C-S Line on the right-hand page, Case IV is applicable.
Extract the tabular altitude which subtracted from 90° is the desired great-circle distance. The tabular azimuth angle is the initial great-circle course angle.
Case Ill-Latitudes of departure and destination of same name and great-circle distance greater than 90°.
Enter the tables with latitude of departure as latitude argument (Same Name), latitude of destination as declination argument, and difference of longitude as local hour angle argument. If the respondents as found on a right-hand page do not lie above the C-S Line, Case I is applicable.
Extract the tabular altitude which added to 90° gives the desired great-circle distance. The initial great-circle course angle is 180° minus the tabular azimuth angle.
Enter the tables with latitude of departure as latitude argument (Same Name), latitude of destination as declination argument, and difference of longitude as local hour angle argument. If the respondents as found on a right-hand page do not lie above the C-S Line, Case I is applicable.
Extract the tabular altitude which added to 90° gives the desired great-circle distance. The initial great-circle course angle is 180° minus the tabular azimuth angle.
Case IV-Latitudes of departure and destination of contrary name and great-circle distance greater than 90 .
Enter the tables with latitude of departure as latitude argument (Contrary Name), latitude of destination as declination argument, and difference of longitude as local hour angle argument. If the respondents as found on a right-hand page do not lie below the C-S Line, Case II is applicable. If the DLo is in excess of 90°, the respondents are found on the facing left-hand page.
Extract the tabular altitude which added to 90° gives the desired great-circle distance. The initial great-circle course angle is 180° minus the tabular azimuth angle.
Enter the tables with latitude of departure as latitude argument (Contrary Name), latitude of destination as declination argument, and difference of longitude as local hour angle argument. If the respondents as found on a right-hand page do not lie below the C-S Line, Case II is applicable. If the DLo is in excess of 90°, the respondents are found on the facing left-hand page.
Extract the tabular altitude which added to 90° gives the desired great-circle distance. The initial great-circle course angle is 180° minus the tabular azimuth angle.
NAUTICAL TERMS AND TRIVIA
1.The sea is foggiest on the? Grand Banks of Newfoundland where on average, 120 days of the year are foggy.
2. What is a nautical "camel"?
Answer: A float used as a fender
Answer: A float used as a fender
3. The "Seven Seas" is a term considered to include "all the waters and oceans of the world".The term has historically been applied to what seven waters?"
Answer: The seven seas are generally considered to be the N. Pacific, S. Pacific, N. Atlantic, S. Atlantic, Arctic, Antarctic, and Indian Oceans.
Answer: The seven seas are generally considered to be the N. Pacific, S. Pacific, N. Atlantic, S. Atlantic, Arctic, Antarctic, and Indian Oceans.
4. What is the brightest star in the sky? Sirius.
5. The term small stuff is used to describe? Line.
6. A mop is called a ? Swab in nautical terms.
7. Caribbean pirates were known as buccaneers. What were Mediterranean pirates called?
Answer: Corsairs.
Answer: Corsairs.
8. In nautical terminology, what is the name given to a light, warm wind on the surface of the sea?
Answer: "Cat's Skin"
Answer: "Cat's Skin"
9. What is the term Scuttlebutt? The cask of drinking water on ships was called a scuttlebutt and since Sailors exchanged gossip when they gathered at the scuttlebutt for a drink of water, scuttlebutt became U.S. Navy slang for gossip or rumors.
10.Binnacle List A ship's sick-list. A binnacle was the stand on which the ship's compass was mounted. In the eighteenth century and probably before, a list was given to the officer or mate of the watch, containing the names of men unable to report for duty. The list was kept at the binnacle.
Thursday, December 27, 2007
CELESTIAL GLOSSARY ( F to Z )
First Difference-the difference between successive tabulations of a quantity.
First Point of Aries -that point of intersection of the ecliptic and the celestial equator occupied by the Sun as it changes from south to north declination on or about March 21. Also called VERNAL EQUINOX.
Geographical Position (GP)-the point where a line drawn from a celestial body to the Earth's center passes through the Earth's surface.
Great Circle-the intersection of a sphere and a plane through its center.
Great-Circle Course-the direction of the great circle through the point of departure and the destination, expressed as angular distance from a reference direction, usually north, to the direction of the great circle. The angle varies from point to point along the great circle. At the point of departure it is called INITIAL GREAT-CIRCLE COURSE.
Greenwich Hour Angle (GHA)-angular distance west of the Greenwich celestial meridian; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the Greenwich celestial meridian and the hour circle of a point on the celestial sphere, measured westward from the Greenwich celestial meridian through 360.
Hour Circle-on the celestial sphere, a great circle through the celestial poles and a celestial body or the vernal equinox. Hour circles are perpendicular to the celestial equator.
Intercept (a)-the difference in minutes of arc between the computed and observed altitudes (corrected sextant altitudes). It is labeled T (toward) or A (away) as the observed altitude is greater or smaller than the computed altitude; Hc greater than Ho, intercept is away (A); Ho greater than Hc, intercept is toward (T).
Line of Position (LOP)-a line indicating a series of possible positions of a craft, determined by observation or measurement.
Local Hour Angle (LHA)-angular distance west of the local celestial meridian; the arc of the celestial equator, or the angle at the celestial pole., between the upper branch of the local celestial meridian and the hour circle of a celestial body or point on the celestial sphere, measured westward from the local celestial meridian through 360.
Meridian Angle (t)-angular distance east or west of the local celestial meridian; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the local celestial meridian and the hour circle of a celestial body, measured eastward or westward from the local celestial meridian through 1800, and labeled E or W to indicate the direction of measurement.
Nadir (Na)-that point on the celestial sphere 180 from the observer's zenith.
Name-the labels Nand S which are attached to latitude and declination are said to be of the same name when they are both N or S and contrary name when one is N and the other is S.
Navigational Triangle-the spherical triangle solved in computing altitude and azimuth and greatcircle sailing problems. The celestial triangle is formed on the celestial sphere by the great circles connecting the elevated pole, zenith of the assumed position of the observer, and a celestial body. The terrestrial triangle is formed on the Earth by the great circles connecting the pole and two places on the Earth: the assumed position of the observer and geographical position of the body for celestialobservations, and the point of departure and destination for great-circle sailing problems. The term' astronomical triangle applies to either the celestial or terrestrial triangle used for solving celestial observations.
Polar Distance (p)-angular distance from a celestial pole; the arc of an hour circle between a celestial pole, usually the elevated pole, and a point on the celestial sphere, measured from the celestial 'pole through 180°.
Prime Meridian-the meridian of longitude 0°, used as the origin for measurement of longitude.
Prime Vertical-the vertical circle through the east and west points of the horizon.
Principal Vertical Circle-the vertical circle through the north and south points of the horizon, coinciding with the celestial meridian.
Respondent-the vaiue in a table or diagram corresponding to the entering arguments. Second
Second Difference-the difference between successive first differences.
Sidereal Hour Angle (SHA)-angular distance west of the vernal equinox; the arc of the celestial equator, or the angle at the celestial pole, between the hour circle of the vernal equinox and the hour circle of a point on the celestial sphere, measured westward from the hour circle of the vernal equinox through 360.
Sight Reduction-the process of deriving from a sight (observation of the altitude, and sometimes also the azimuth, of a celestial body) the information needed for establishing a line of position.
Small Circle-the intersection of a sphere and a plane which does not pass through its center.
Vertical Circle-on the celestial sphere, a great circle through the zenith and nadir. Vertical circles are perpendicular to the horizon.
Zenith (Z)-that point on the celestial sphere vertically overhead.
Zenith Distance (z)-angular distance from the zenith; the arc of a vertical circle between the zenith and a point on the celestial sphere.
CELESTIAL GLOSSARY ( A to E )
Altitude-the arc of a vertical circle between the horizon and a point or body on the celestial sphere. Altitude as measured by a.sextant is called sextant altitude (hs). Sextant altitude corrected only for inaccuracies in the reading (instrument, index, and personal errors, as applicable) and inaccuracies in the reference level (principally dip) is called apparent altitude (ha). After all corrections are applied, it is called corrected sextant altitude or observed altitude (Ho). An altitude taken directly from a table is called a tabular or tabulated altitude (ht). Tabular altitude as interpolated for declination, latitude, and LHA increments as required is called computed altitude (Hc).
Altitude Difference (d)-the first difference between successive tabulations of altitude in a latitude column of these tables.
Argument-one of the values used for entering a table or diagram.
Assumed (or Chosen) Latitude (aL), Assumed (or Chosen) Longitude (a)-geographical coordinates assumed to facilitate sight reduction.
Assumed Position (AP)-a point at which an observer is assumed to be located.
Azimuth (Zn)-the horizontal direction of a celestial body or point from a terrestrial point; the arc of the horizon, or the angle at the zenith, between the north part of the celestial meridian or principal vertical circle and a vertical circle through the body or point, measured from 000 at the north part of the principal vertical circle clockwise through 360°.
Azimuth Angle (Z)-the arc of the horizon, or the angle at the zenith, between the north part or south part of the celestial meridian, according to the elevated pole, and a vertical circle through the body or point, measured from 0° at the north or south reference eastward or westward through 180° according to whether the body is east or west of the local meridian. It is prefixed N or S to agree with the latitude and suffixed E or W to agree with the meridian angle.
Celestial Equator-the primary great circle of the celestial sphere, everywhere 90° from the celestial poles; the intersection of the extended plane of the equator and the celestial sphere. Also called EQUINOCTIAL.
Celestial Horizon-that circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through the center of the Earth and perpendicular to zenith-nadir line.
Celestial Meridian-on the celestial sphere, a great circle through the celestial poles and the zenith. The expression usually refers to the upper branch, that half from pole to pole which passes through the zenith.
Course Angle-course measured from 0° at the reference direction clockwise or counterclockwise through 180°. It is labeled with the reference direction as a prefix and the direction of measurement from the reference direction as a suffix. Course angle S21°E is 21 ° east of south, or true course 159°.
Course Line-the graphic representation of a ship's course.
Declination (Dec.)-angular distance north or south of the celestial equator; the arc of an hour circle between the celestial equator and a point on the celestial sphere, measured northward or southward from the celestial equator through 90°, and labeled N or S (+ or -) to indicate the direction of measurement.
Declination Increment (Dec. Inc.)-in sight reduction, the excess of the actual declination of a celestial body over the integral declination argument.
Double-Second Difference (DSD)-the sum of successive second differences. Because second differences are not tabulated in these tables, the DSD can be formed most readily by subtracting, algebraically, the first difference immediately above the tabular -altitude difference (d) corresponding to the entering arguments from the first difference immediately below. The result will always be a negative value.
Ecliptic-the apparent armual path of the Sun among the stars; the intersection of the plane of the Earth's orbit with the celestial sphere. This is a great circle of the celestial sphere inclined at an angle of about 23° 27' to the celestial equator.
Elevated Pole (Pn or Ps)-the celestial pole above the observer's horizon, agreeing in name with the observer's latitude.
Altitude Difference (d)-the first difference between successive tabulations of altitude in a latitude column of these tables.
Argument-one of the values used for entering a table or diagram.
Assumed (or Chosen) Latitude (aL), Assumed (or Chosen) Longitude (a)-geographical coordinates assumed to facilitate sight reduction.
Assumed Position (AP)-a point at which an observer is assumed to be located.
Azimuth (Zn)-the horizontal direction of a celestial body or point from a terrestrial point; the arc of the horizon, or the angle at the zenith, between the north part of the celestial meridian or principal vertical circle and a vertical circle through the body or point, measured from 000 at the north part of the principal vertical circle clockwise through 360°.
Azimuth Angle (Z)-the arc of the horizon, or the angle at the zenith, between the north part or south part of the celestial meridian, according to the elevated pole, and a vertical circle through the body or point, measured from 0° at the north or south reference eastward or westward through 180° according to whether the body is east or west of the local meridian. It is prefixed N or S to agree with the latitude and suffixed E or W to agree with the meridian angle.
Celestial Equator-the primary great circle of the celestial sphere, everywhere 90° from the celestial poles; the intersection of the extended plane of the equator and the celestial sphere. Also called EQUINOCTIAL.
Celestial Horizon-that circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through the center of the Earth and perpendicular to zenith-nadir line.
Celestial Meridian-on the celestial sphere, a great circle through the celestial poles and the zenith. The expression usually refers to the upper branch, that half from pole to pole which passes through the zenith.
Course Angle-course measured from 0° at the reference direction clockwise or counterclockwise through 180°. It is labeled with the reference direction as a prefix and the direction of measurement from the reference direction as a suffix. Course angle S21°E is 21 ° east of south, or true course 159°.
Course Line-the graphic representation of a ship's course.
Declination (Dec.)-angular distance north or south of the celestial equator; the arc of an hour circle between the celestial equator and a point on the celestial sphere, measured northward or southward from the celestial equator through 90°, and labeled N or S (+ or -) to indicate the direction of measurement.
Declination Increment (Dec. Inc.)-in sight reduction, the excess of the actual declination of a celestial body over the integral declination argument.
Double-Second Difference (DSD)-the sum of successive second differences. Because second differences are not tabulated in these tables, the DSD can be formed most readily by subtracting, algebraically, the first difference immediately above the tabular -altitude difference (d) corresponding to the entering arguments from the first difference immediately below. The result will always be a negative value.
Ecliptic-the apparent armual path of the Sun among the stars; the intersection of the plane of the Earth's orbit with the celestial sphere. This is a great circle of the celestial sphere inclined at an angle of about 23° 27' to the celestial equator.
Elevated Pole (Pn or Ps)-the celestial pole above the observer's horizon, agreeing in name with the observer's latitude.
Wednesday, December 26, 2007
WEATHER SAYINGS
First Step in Weather Casting: Step outside and look at the sky!
Red sky at night, sailor's delight; Red sky in the morning, sailors take warning.
Red sky at night, sailor's delight; Red sky in the morning, sailors take warning.
The evening red and morning gray are sure signs of a fine day, But the evening gray and the morning red, makes the sailor shake his head.
These expressions have some basis in meteorological fact. In the mid latitudes of the Northern Hemisphere, storms generally travel with the jetstream from west to east. A red sky in the morning may indicate that is rising in clear eastern skies and casting its rays on storm clouds approaching from the west. A red sky at sunset indicates clouds to the east, with clear skies moving in from the west, allowing the sunset to be seen.
These expressions have some basis in meteorological fact. In the mid latitudes of the Northern Hemisphere, storms generally travel with the jetstream from west to east. A red sky in the morning may indicate that is rising in clear eastern skies and casting its rays on storm clouds approaching from the west. A red sky at sunset indicates clouds to the east, with clear skies moving in from the west, allowing the sunset to be seen.
A ring around the sun or moon, means rain or snow coming soon.
If a circle forms around the moon, Twill rain soon.
The circle that forms around the sun or moon is called a halo. Halos are formed by the light from the sun or moon refracting (bending) as they pass through the ice crystals that form high-level cirrus and cirrostratus clouds. These clouds do not produce rain or snow, but they often precede an advancing low pressure system which may bring bad weather. This form of weather prediction is accurate about 65% of the time.
If a circle forms around the moon, Twill rain soon.
The circle that forms around the sun or moon is called a halo. Halos are formed by the light from the sun or moon refracting (bending) as they pass through the ice crystals that form high-level cirrus and cirrostratus clouds. These clouds do not produce rain or snow, but they often precede an advancing low pressure system which may bring bad weather. This form of weather prediction is accurate about 65% of the time.
Rainbow to windward, foul fall the day rainbow to leeward, rain runs away.
A windward rainbow indicates rain upwind, so it may begin raining soon. A rainbow behind the wind or to leeward implies the rain has probably past.
A windward rainbow indicates rain upwind, so it may begin raining soon. A rainbow behind the wind or to leeward implies the rain has probably past.
Sun sets Friday clear as bell, rain on Monday sure as hell.
Cold is the night When the stars shine bright.
The more moisture there is in the sky, the more the light from the sun, moon, and stars is dimmed or reddened. A very clear sky permits more starlight to penetrate, thus the stars appear brighter. Moisture tends to hold in the day's heat, like a blanket. The less moisture there is in the air at night, the more the temperature tends to fall. The brighter the stars appear, the cooler is the
Cold is the night When the stars shine bright.
The more moisture there is in the sky, the more the light from the sun, moon, and stars is dimmed or reddened. A very clear sky permits more starlight to penetrate, thus the stars appear brighter. Moisture tends to hold in the day's heat, like a blanket. The less moisture there is in the air at night, the more the temperature tends to fall. The brighter the stars appear, the cooler is the
night.
If clouds are gathering thick and fast, Keep sharp look out for sail and mast, But if they slowly onward crawl, shoot your lines, nets and trawl.
In the morning mountains. In the evening fountains.
The mountains refer to high, billowing cumulus clouds, indicative of instability and possible development of cumulonimbus clouds and a late afternoon or evening thunderstorm.
In the morning mountains. In the evening fountains.
The mountains refer to high, billowing cumulus clouds, indicative of instability and possible development of cumulonimbus clouds and a late afternoon or evening thunderstorm.
Step Two for the Weather Man: Check to see from which quarter the wind blows!
"A wind in the south has rain in her mouth."
A southerly wind usually carries moisture from the Gulf of Mexico. It causes the air to become more humid, and, more likely to form rain clouds.
When the wind is blowing in the North, no fisherman should set forth, When the wind is blowing in the East, 'Tis not fit for man nor beast, When the wind is blowing in the South it brings the food over the fish's mouth, When the wind is blowing in the West, That is when the fishing's best!
"A wind in the south has rain in her mouth."
A southerly wind usually carries moisture from the Gulf of Mexico. It causes the air to become more humid, and, more likely to form rain clouds.
When the wind is blowing in the North, no fisherman should set forth, When the wind is blowing in the East, 'Tis not fit for man nor beast, When the wind is blowing in the South it brings the food over the fish's mouth, When the wind is blowing in the West, That is when the fishing's best!
When rain comes before the wind, halyards, sheets and braces mind, but when wind comes before rain, soon you may make sail again.
No weather's ill if the wind be still.
No weather's ill if the wind be still.
Weather Casting Rule Number Three: If you don't trust your own judgement, check with the local wildlife!
Sea gull, sea gull, sit on the sand, It's never good weather while you're on the land. or when sea-gulls fly to land, a storm is at hand.
Sea gull, sea gull, sit on the sand, It's never good weather while you're on the land. or when sea-gulls fly to land, a storm is at hand.
Sharks go out to sea at the approach of a wave of cold weather.
When porpoises sport and play, there will be a storm.
When parrots whistle, expect rain.
Failing All That, Choose Step Number Four: The most important weather rule of all for sailors!
Whether the weather be fine Or whether the weather be not Whether the weather be cold Or whether the weather be hot We'll weather the weather Whatever the weather Whether we like it or not.
When porpoises sport and play, there will be a storm.
When parrots whistle, expect rain.
Failing All That, Choose Step Number Four: The most important weather rule of all for sailors!
Whether the weather be fine Or whether the weather be not Whether the weather be cold Or whether the weather be hot We'll weather the weather Whatever the weather Whether we like it or not.
NAUTICAL TRIVIA
Questions:
1. How big is the Pacific Ocean in Square Miles?
2.What was the first lighthouse built in America?
3.What is the oldest continuously operating lighthouse on the Great Lakes?
4.What is the definition of "Footloose"?
5.What is the most dangerous sea creature?
6.What is the state that is named the source of the Mississippi River?
7.Which canal is the largest Suez, Panama or Cape Cod?
8.What is the fastest fish in the ocean?
9.What percent of boats sold are used for fishing?
10.What is the average cost to tow a boat if it breaks down on the water?
Answers:
1.63,800,000
2.Boston Light
3.Marblehead Light
4.When bottom portion of the sail is not attached, it wildly swings (dances) around.
5.The Sea Wasp
6.Minnesota
7.The Panama Canal
8.Sailfish: 68.18 mph!!!!!
9.70%
10.$260
1. How big is the Pacific Ocean in Square Miles?
2.What was the first lighthouse built in America?
3.What is the oldest continuously operating lighthouse on the Great Lakes?
4.What is the definition of "Footloose"?
5.What is the most dangerous sea creature?
6.What is the state that is named the source of the Mississippi River?
7.Which canal is the largest Suez, Panama or Cape Cod?
8.What is the fastest fish in the ocean?
9.What percent of boats sold are used for fishing?
10.What is the average cost to tow a boat if it breaks down on the water?
Answers:
1.63,800,000
2.Boston Light
3.Marblehead Light
4.When bottom portion of the sail is not attached, it wildly swings (dances) around.
5.The Sea Wasp
6.Minnesota
7.The Panama Canal
8.Sailfish: 68.18 mph!!!!!
9.70%
10.$260
Tuesday, December 25, 2007
SELECTING BODIES FOR OBSERVATION
SELECTING BODIES FOR OBSERVATION
Before going further into problems and tables, mention should be made of a few items concerned with selecting astronomical bodies for observation.
Observing two heavenly bodies in rapid succession is the most convenient method of finding two lines of position necessary to establish a fix. Noting three bodies gives three lines, and these three define the fix more accurately (as in piloting). Accuracy of the fix established by intersecting lines of position depends upon the angle between the lines. The nearer this angle approaches 90°, the more accurate is the fix.
Actually, sights seldom are taken on two or more bodies simultaneously. Instead, the navigator decides which bodies to observe, then takes a round of sights, each one timed exactly. Resulting lines of position are advanced or retarded the amount of the ship's run between the time of observation and the time of the desired fix. The ideal situation for lines of position established by observing three bodies would be that wherein the bodies lie 120° apart in azimuth. An ideal fix using four bodies would include two north - south lines and two east - west lines of position to form a box. As already mentioned, lines , perpendicular to the course are frequently valuable for checking the run. Those lines parallel to it are helpful in deciding the accuracy of the course made good.
Concerning altitude, best results are obtained by observation of bodies whose altitudes are between 15° and 65° In general, observations are taken from bodies whose altitudues are between 10° and 80°.
ADVANCING AND RETARDING THE LOP
Several methods may be used to advance a fine of position. The most frequent method consists of advancing the AP in the direction of your course and for the distance of the run, and drawing the new LOP. To retard a LOP, just go the reciprocal of your course and for the distance run.
If appreciable time has elapsed since the determination of the last fix of the Ship's position at sea, the error in the DR plot may change where the ship's actual position is well away from her DR plot. A single line of position can be useful in establishing an estimated position. If an accurate line is obtained, the actual position is somewhere on this line. In the absence of better information, a perpendicular from the previous DR position or EP to the line of position establishes the new EP. The foot of the perpendicular from the AP has no significance in this regard, since it is used only to locate the line of position.
The establishment of a good EP is dependent upon accurate interpretation of all information available. Generally, such ability can be acquired only by experience. If, in the judgment of the navigator, the course has been made good, but the speed has been uncertain, the best estimate of the position might be at the intersection of the course line and the LOP. If the speed since the last fix is considered accurate, but the course is considered uncertain, the EP might be at the intersection of the line of position and an arc centered on the previous fix and of radius equal to distance traveled.
Before going further into problems and tables, mention should be made of a few items concerned with selecting astronomical bodies for observation.
Observing two heavenly bodies in rapid succession is the most convenient method of finding two lines of position necessary to establish a fix. Noting three bodies gives three lines, and these three define the fix more accurately (as in piloting). Accuracy of the fix established by intersecting lines of position depends upon the angle between the lines. The nearer this angle approaches 90°, the more accurate is the fix.
Actually, sights seldom are taken on two or more bodies simultaneously. Instead, the navigator decides which bodies to observe, then takes a round of sights, each one timed exactly. Resulting lines of position are advanced or retarded the amount of the ship's run between the time of observation and the time of the desired fix. The ideal situation for lines of position established by observing three bodies would be that wherein the bodies lie 120° apart in azimuth. An ideal fix using four bodies would include two north - south lines and two east - west lines of position to form a box. As already mentioned, lines , perpendicular to the course are frequently valuable for checking the run. Those lines parallel to it are helpful in deciding the accuracy of the course made good.
Concerning altitude, best results are obtained by observation of bodies whose altitudes are between 15° and 65° In general, observations are taken from bodies whose altitudues are between 10° and 80°.
ADVANCING AND RETARDING THE LOP
Several methods may be used to advance a fine of position. The most frequent method consists of advancing the AP in the direction of your course and for the distance of the run, and drawing the new LOP. To retard a LOP, just go the reciprocal of your course and for the distance run.
If appreciable time has elapsed since the determination of the last fix of the Ship's position at sea, the error in the DR plot may change where the ship's actual position is well away from her DR plot. A single line of position can be useful in establishing an estimated position. If an accurate line is obtained, the actual position is somewhere on this line. In the absence of better information, a perpendicular from the previous DR position or EP to the line of position establishes the new EP. The foot of the perpendicular from the AP has no significance in this regard, since it is used only to locate the line of position.
The establishment of a good EP is dependent upon accurate interpretation of all information available. Generally, such ability can be acquired only by experience. If, in the judgment of the navigator, the course has been made good, but the speed has been uncertain, the best estimate of the position might be at the intersection of the course line and the LOP. If the speed since the last fix is considered accurate, but the course is considered uncertain, the EP might be at the intersection of the line of position and an arc centered on the previous fix and of radius equal to distance traveled.
DETERMINING A LINE OF POSITION
LINE OF POSITION
In practice, you may neither be able nor will you need to plot the whole of a circle of equal altitude. The position is usually known within 10 miles and possibly even less than that. Inside these limits, the curve of the arc of a circle of equal altitude is hardly perceptible, and the arc is plotted and regarded as a straight line. Such a line, comprising enough of the arc of a circle of equal altitude to cover the probable limits of a position, is called a Sumner line of position or just a line of position.
TWO LINES OF POSITION
The preferable method of establishing two lines of position is by observing two different bodies, although two lines may be obtained from the same body by observations taken at different times. As in piloting, the nearer the two lines approach a right angle to each other, the more accurate is the fix.
When two lines are determined by observing the same body, the first line established is brought forward the distance run on the course steered. For example, if a ship steams 27 miles on course 315° between the first and second observations, obviously her position is on a line parallel with the first one established, but drawn 27 miles away (to scale) on the course line 315°. Intersection of the line established by the second observation with the advanced line of the first observation is a fix. The fix progressively decreases in accuracy, depending on how, far the first line is advanced. You should not advance such a time for more than 5 hours of run.
DETERMINING A LINE OF POSITION
At this point you might be entitled to complain that much has been said concerning what a line of position tells you, but very little has been said about how you should determine it in the first place. We are coming to that part now.
You probably have grasped the idea that what you want to find out is which circle of equal altitude you are on, and what this altitude is. To draw such a circle, you would need a chart covering an extensive area, unless the heavenly body's altitude approached 90. Consequently, you do not determine the entire circle but merely a portion of its arc, so small that it is plotted and regarded as a straight line.
An assumed position (AP) is selected according to the rule of 30' of your DR position for the time of sight. Observation of a star provides sextant altitude. Sextant altitude is then corrected to obtain observed altitude (ho). The star's altitude from the assumed position (called the computed altitude (Hc)) and its azimuth angle are determined from tables and the azimuth angle is then converted to azimuth.
After selecting an AP, draw the azimuth through the AP. Along the azimuth; measure off the altitude intercept (difference between the observed altitude and the computed altitude). At the end of this measurement, draw a perpendicular line, which is the LOP.
You must know whether altitude intercept (a) should be measured from AP TOWARD the star or from AP AWAY from the star. (Frequently, the initials for Coast Guard Academy (CGA) are found to be helpful.) If the computed altitude is greater than the observed altitude, altitude intercept (a) is measured away from the star. (In other words applying the CGA "memory aid," you have computed. greater, away (CGA).)
In practice, you may neither be able nor will you need to plot the whole of a circle of equal altitude. The position is usually known within 10 miles and possibly even less than that. Inside these limits, the curve of the arc of a circle of equal altitude is hardly perceptible, and the arc is plotted and regarded as a straight line. Such a line, comprising enough of the arc of a circle of equal altitude to cover the probable limits of a position, is called a Sumner line of position or just a line of position.
TWO LINES OF POSITION
The preferable method of establishing two lines of position is by observing two different bodies, although two lines may be obtained from the same body by observations taken at different times. As in piloting, the nearer the two lines approach a right angle to each other, the more accurate is the fix.
When two lines are determined by observing the same body, the first line established is brought forward the distance run on the course steered. For example, if a ship steams 27 miles on course 315° between the first and second observations, obviously her position is on a line parallel with the first one established, but drawn 27 miles away (to scale) on the course line 315°. Intersection of the line established by the second observation with the advanced line of the first observation is a fix. The fix progressively decreases in accuracy, depending on how, far the first line is advanced. You should not advance such a time for more than 5 hours of run.
DETERMINING A LINE OF POSITION
At this point you might be entitled to complain that much has been said concerning what a line of position tells you, but very little has been said about how you should determine it in the first place. We are coming to that part now.
You probably have grasped the idea that what you want to find out is which circle of equal altitude you are on, and what this altitude is. To draw such a circle, you would need a chart covering an extensive area, unless the heavenly body's altitude approached 90. Consequently, you do not determine the entire circle but merely a portion of its arc, so small that it is plotted and regarded as a straight line.
An assumed position (AP) is selected according to the rule of 30' of your DR position for the time of sight. Observation of a star provides sextant altitude. Sextant altitude is then corrected to obtain observed altitude (ho). The star's altitude from the assumed position (called the computed altitude (Hc)) and its azimuth angle are determined from tables and the azimuth angle is then converted to azimuth.
After selecting an AP, draw the azimuth through the AP. Along the azimuth; measure off the altitude intercept (difference between the observed altitude and the computed altitude). At the end of this measurement, draw a perpendicular line, which is the LOP.
You must know whether altitude intercept (a) should be measured from AP TOWARD the star or from AP AWAY from the star. (Frequently, the initials for Coast Guard Academy (CGA) are found to be helpful.) If the computed altitude is greater than the observed altitude, altitude intercept (a) is measured away from the star. (In other words applying the CGA "memory aid," you have computed. greater, away (CGA).)
Monday, December 24, 2007
HOW BRIGHT ARE THE STARS
The stars visible to the naked eye range more than a thousandfold in brightness, from the most brilliant one, Sirius, to those that can only just be glimpsed on the darkest of nights. Astronomers term a star's brightness its magnitude. The magnitude system is one of the odder conventions of astronomy.
Naked-eye stars are ranked in six magnitude classes, from first magnitude (the brightest) to sixth magnitude (the faintest). A difference of five magnitudes is defined as equalling a brightness difference of exactly 100 times. Hence a step of one magnitude corresponds to a difference of about 2.5 times in brightness. A difference of two magnitudes corresponds to a brightness difference of 2.5 X 2.5 = 6.3 times. Three magnitudes equals a brightness difference of 2.5 x 2.5 x 2.5 = 16 times, and so on.
A star 2.5 times brighter than magnitude 1.0 is said to be of magnitude 0. Objects brighter still are assigned negative magnitudes. Sirius, the brightest star in the sky, has a magnitude of -1.46.
The magnitude system can be extended indefinitely to take account of the brightest and the faintest objects. Example, the Sun has a magnitude of -27. Objects fainter than sixth magnitude are classified in succession as seventh magnitude, eighth magnitude, and so on. The faintest objects that can be detected by telescopes on Earth are about magnitude 25.
Naked-eye stars are ranked in six magnitude classes, from first magnitude (the brightest) to sixth magnitude (the faintest). A difference of five magnitudes is defined as equalling a brightness difference of exactly 100 times. Hence a step of one magnitude corresponds to a difference of about 2.5 times in brightness. A difference of two magnitudes corresponds to a brightness difference of 2.5 X 2.5 = 6.3 times. Three magnitudes equals a brightness difference of 2.5 x 2.5 x 2.5 = 16 times, and so on.
A star 2.5 times brighter than magnitude 1.0 is said to be of magnitude 0. Objects brighter still are assigned negative magnitudes. Sirius, the brightest star in the sky, has a magnitude of -1.46.
The magnitude system can be extended indefinitely to take account of the brightest and the faintest objects. Example, the Sun has a magnitude of -27. Objects fainter than sixth magnitude are classified in succession as seventh magnitude, eighth magnitude, and so on. The faintest objects that can be detected by telescopes on Earth are about magnitude 25.
HOW FAR ARE THE STARS
So remote are the stars that their distances are measured not in kilometres or miles but in the time that light takes to travel from them to us. Light has the fastest speed in the Universe, 300,000 sec (l86,000 mile). It takes just over 1 second to cross the gap from the Moon to the Earth, 8.3 minutes to reach us from the Sun, and 4.3 years to reach the Earth from the nearest star. Alpha Centauri. Hence Alpha Centauri is 4.3 years away.
Most of the stars visible to the naked eye lie from dozons to hundreds of light years away. It is hard to think that the light entering our eyes at night left those stars so long ago. The most distant stars that can be seen by the naked eye are over 1000 light years away, for example Deneb in the constellation Cygnus and several of the stars in Orion. Only the most luminous stars, those that blaze more brightly than 50,000 Suns, are visible to the naked eye over such great distances. At the other end of the scale, the feeblest stars emit less than a thousandth the light of the Sun, and even the closest of them cannot be seen without a telescope.
Sunday, December 23, 2007
Saturday, December 22, 2007
PIRATES (HAVING A GOOD TIME)
Pirates were happiest when they had plenty of money to spend and were enjoying themselves on dry land. Their life at sea was very hard, but sometimes there was fun to be had onboard ship too. Singing, for example, helped with repetitive work as well as providing entertainment
AND A BOTTLE OF RUM Alot pirates wasted all their money when they reached land, on drinking and gambling. The idea that most pirates buried their treasure on desert islands is probably untrue, for almost all of them would have spent everything they had on shore. Port Royal, in Jamaica, was a favorite spot amongst pirates, and it gained a reputation as the wickedest city in the world. It had so many taverns that there was one for every 150 people. When the city was devastated by an earthquake in 1692, many said it was God's punishment on the place.
GOING ASHORE Pirates sometimes plundered the cities they passed through when they went ashore, but usually when pirates reached land they came to stock up on food and medicines, make repairs to their ships, and enjoy themselves. Blackbeard frequently got married! Some say he had fourteen wives in different ports.
REPAIRING SHIPS
Wooden ships had to be carefully maintained to keep them seaworthy. Every so often the ship had to be "careened" put onto its side so that weed and barnacles could be scraped off the bottom. Weed slowed ships down, and pirate ships had to be fast in case they were chased, Pirates found quiet inlets to repair their ships.
Wooden ships had to be carefully maintained to keep them seaworthy. Every so often the ship had to be "careened" put onto its side so that weed and barnacles could be scraped off the bottom. Weed slowed ships down, and pirate ships had to be fast in case they were chased, Pirates found quiet inlets to repair their ships.
PIRATES (MAKING SILVER PIECES EIGHT)
PIECES OF EIGHT
Most seamen became pirates because they hoped to become rich, but pirate plunder was only valuable when exchanged for cash. Pirates were overjoyed if they captured a ship carrying money, not least because coins w~re more easily shared amongst the crew. In storybooks, pirates mostly seem to deal in pieces of eight. These were Spanish in origin, and worth about one dollar. Between the 17th and 19th centuries, they were accepted as currency almost worldwide.
HUMAN TREASURE
The Barbary corsairs, operating from the North African coast, found that more money could be made by taking prisoners hostage than by seeking rich cargoes. Prisoners were ransomed or sold as slaves (sometimes sent to row in galley ships).
Most seamen became pirates because they hoped to become rich, but pirate plunder was only valuable when exchanged for cash. Pirates were overjoyed if they captured a ship carrying money, not least because coins w~re more easily shared amongst the crew. In storybooks, pirates mostly seem to deal in pieces of eight. These were Spanish in origin, and worth about one dollar. Between the 17th and 19th centuries, they were accepted as currency almost worldwide.
HUMAN TREASURE
The Barbary corsairs, operating from the North African coast, found that more money could be made by taking prisoners hostage than by seeking rich cargoes. Prisoners were ransomed or sold as slaves (sometimes sent to row in galley ships).
BURIED TREASURE
Most pirates did not bury their treasure and leave behind a map. Instead they squandered their loot on women, gambling, and drinking. Supposedly, Captain Kidd did bury his treasure on Gardiner's Island, New York, but he didn't live to reclaim it and it has never been found.
French pirate Olivier Ie Vasseur (alias The Buzzard), is also said to have buried a fortune. In 1721 he seized a Portuguese ship carrying gold and silver bars, chests of gold guineas, casks of diamonds, silks, and luxury goods worth a fortune. The Buzzard was captured alive in 1730. At his hanging he is said to have flung a roll of documents at the crowd with the challenge, "Find my treasure, who can!
Most pirates did not bury their treasure and leave behind a map. Instead they squandered their loot on women, gambling, and drinking. Supposedly, Captain Kidd did bury his treasure on Gardiner's Island, New York, but he didn't live to reclaim it and it has never been found.
French pirate Olivier Ie Vasseur (alias The Buzzard), is also said to have buried a fortune. In 1721 he seized a Portuguese ship carrying gold and silver bars, chests of gold guineas, casks of diamonds, silks, and luxury goods worth a fortune. The Buzzard was captured alive in 1730. At his hanging he is said to have flung a roll of documents at the crowd with the challenge, "Find my treasure, who can!
MAKING SILVER PIECES OF EIGHT
Pirates liked to enjoy themselves when they went ashore, and were not noted for their savings. Certainly very few of them held onto their money long enough to bury any of it.
What you will need:
· lids or pastry cutters to draw around to make small circles.
· cardboard
· cork
· craft knife (TAKE CARE)
· scissors
· aluminum and gold foil
· pencil
What you will need:
· lids or pastry cutters to draw around to make small circles.
· cardboard
· cork
· craft knife (TAKE CARE)
· scissors
· aluminum and gold foil
· pencil
1. Draw around lids or pastry cutters on a cardboard.
2. Cut the discs out, for smaller coins, slice off pieces from a cork.
3. Cover all the pieces with silver or gold foil, with the shiny side facing out.
4. Draw designs on top, using a pencil. (Take care not to pierce through the foil.)
HOW TO MAKE A PIRATE FLAG
To make your own flag and design, decide what meaning your symbols will have to those you capture.
What you will need:
A round wooden rod (24 in.) in length and (1/8 in.) in diameter.
· 1 picture hanging hoop screw or a ring pull from a soft-drink can.
· masking tape.
· black material or black plastic.
· scissors.
· string (double the length of your rod)
· plain white paper
· pencil
· fluorescent paint or pen
· glue
What you will need:
A round wooden rod (24 in.) in length and (1/8 in.) in diameter.
· 1 picture hanging hoop screw or a ring pull from a soft-drink can.
· masking tape.
· black material or black plastic.
· scissors.
· string (double the length of your rod)
· plain white paper
· pencil
· fluorescent paint or pen
· glue
1. Screw the hoop screw into the wooden rod, about (1/4 in.) from one end, or use a ring-pull from a soft-drink can, attached firmly to the side of the rod with masking tape.
2. Cut a rectangle out of black material or plastic (16 x 40 in.). Glue one of the shorter edges around the middle of the length of string so it is stuck securely.
3. On paper, draw your design for a flag maybe a skull, or some other symbol. Color it (fluorescent colors are great) and cut it out. Glue it to your black flag.
4. Thread the string above the flag down through the screw - ring-pull and tie it to the other end of the string. Hoist the flag. Wrap the string around the pole to keep it secured.
4. Thread the string above the flag down through the screw - ring-pull and tie it to the other end of the string. Hoist the flag. Wrap the string around the pole to keep it secured.
PIRATE FLAGS
Traditionally, a ship flies the flag of the country that owns her, but not pirate ships. Sometimes pirates deceived passing vessels by flying the flag of a friendly country, but they also flew their own flags. The most famous of was the skull and crossbones of The Jolly Roger.
FLAGS, WHITE AND RED
Pirates built-up a reputation for cruelty and violence. They used flags to frighten passing ships, hoping that they would surrender without too much of a fight. When giving chase, pirates often flew a white flag.
If the merchant ship refused to slow down, the pirates hoisted a red flag. The red flag signified blood. The message it sent to the resisting ship was that once the pirates boarded, no one would be spared.
Pirates built-up a reputation for cruelty and violence. They used flags to frighten passing ships, hoping that they would surrender without too much of a fight. When giving chase, pirates often flew a white flag.
If the merchant ship refused to slow down, the pirates hoisted a red flag. The red flag signified blood. The message it sent to the resisting ship was that once the pirates boarded, no one would be spared.
THE JOLLY ROGER
The skull and crossbones flag is the most famous symbol of pirate terror. The first Jolly Roger appeared around 1700 when the pirate Emmanuel Wynne hoisted one in the Caribbean. The flag quickly caught on and other pirates designed their own versions. The Jolly Roger was flown when pirates were close to their victims and wanted to frighten them badly. No one knows the origin of the name. It may have come from the French joli rouge (pretty red), a joke description of the blood-red flag flown by earlier pirates.
HOW TO MAKE A TREASURE MAP
What you will need:
· medium-weight plain paper
· colored pencils or waterproof pens
· a used, slightly damp teabag
· ribbon
· colored, bakeable modeling clay
· old or new coin( s), as big as possible
· medium-weight plain paper
· colored pencils or waterproof pens
· a used, slightly damp teabag
· ribbon
· colored, bakeable modeling clay
· old or new coin( s), as big as possible
1. Draw a map of a desert island. Decide where your camp is. Where do you go to fish for food? Where are the swampy places? And where would you bury any treasure?
Put on as many sites as you like.
Put on as many sites as you like.
2. Make your map look old. Bend the corners and tear them a little. Use the teabag (not too damp) to wipe over the drawing. Leave to dry.
3. You could roll up the map, tie ribbon around the outside, and add a seal. Seals can be made from pressing a coin into modeling clay. Remove the coin and bake the clay for about 20 minutes. Cut a small "V" out of one end of the ribbon and stick the seal close to the other end. Alternatively, you could cut a line at the bottom of your map, and thread your ribbon this, with the seal hanging at the front.
PIRATES CHRISTMAS DINNER
If pirates were blown off course, or failed to take any ships, then they could easily run out of food and get very, very hungry. Pirates rarely ate a healthy diet, and without the vitamin C to be found in fresh fruit and vegetables they were likely to catch a disease called scurvy.
PIRATES' COOKERY SPOT Salmagundi was a favorite pirate meal when food was plentiful. The pirate Bartholomew Roberts ate it for breakfast on the day that he died, when trying to avoid capture by a British warship in 1722. Perhaps he ate a bit too much and couldn't get away!
Here's how to make it:
Salmagundi: A light snack for a hungry pirate
Here's how to make it:
Salmagundi: A light snack for a hungry pirate
INGREDIENTS:
Meats: 1 turtle, 1 fish, 1 chicken, 1 pig, 1 cow, 1 duck, 1 pigeon.
Marinade: Red wine, spices. Accompaniments: Cabbage, pickled herring, anchovies, mangoes, onions, grapes, eggs (hardboiled), pickled vegetables.
Meats: 1 turtle, 1 fish, 1 chicken, 1 pig, 1 cow, 1 duck, 1 pigeon.
Marinade: Red wine, spices. Accompaniments: Cabbage, pickled herring, anchovies, mangoes, onions, grapes, eggs (hardboiled), pickled vegetables.
Seasonings: Garlic, salt, pepper, mustard seed, oil, vinegar.
For this spicy West Indian dish, first roast all the meats. Cut into
chunks and marinate for several hours in spiced red wine. Remove, and mix with all remaining ingredients, having first chopped them into bite-sized pieces. Season to taste.
For this spicy West Indian dish, first roast all the meats. Cut into
chunks and marinate for several hours in spiced red wine. Remove, and mix with all remaining ingredients, having first chopped them into bite-sized pieces. Season to taste.
SHIP'S BISCUIT
Bread did not keep long at sea, so instead pirates ate plain biscuits, made of flour and very little water. Shaped into flat cakes, they were baked very slowly and then packed for storage in canvas bags. They quickly became infested with black-headed weevils, which had to be taken out before the biscuits could be eaten. For a tastier version for today's seafarers, try this biscuit recipe:
Bread did not keep long at sea, so instead pirates ate plain biscuits, made of flour and very little water. Shaped into flat cakes, they were baked very slowly and then packed for storage in canvas bags. They quickly became infested with black-headed weevils, which had to be taken out before the biscuits could be eaten. For a tastier version for today's seafarers, try this biscuit recipe:
INGREDIENTS:
8 oz plain flour 5 teaspoons baking soda 1 teaspoon salt
2 teaspoons sugar
4 oz butter 2 fl oz milk
8 oz plain flour 5 teaspoons baking soda 1 teaspoon salt
2 teaspoons sugar
4 oz butter 2 fl oz milk
1. Sift the dry ingredients together into a mixing bowl. Using your fingertips, work the butter well into the mixture.
2. Stir in just enough milk to make a smooth, soft dough - not too sticky to be handled. Turn out onto a lightly-floured work surface and knead gently for about a minute.
3. Roll the dough out with a lightly floured rolling pin, to between a (1/4 - 1/2 in.) thick.
4. Use a round cutter (or the floured top of a glass) to cut out your biscuits. Arrange on a buttered baking sheet.
5. Bake at 450° for 12-15 minutes, or until lightly browned and done. Serve with butter and jam.
HAVE YOU GOT SCURVY?
Take this quiz to discover if you must see a doctor and be bled by leeches at the next port.
1.Do you have any of the following?
Sore gums (Score A)
Rumbling stomach (Score B)
Sore gums (Score A)
Rumbling stomach (Score B)
Itchy feet (Score C)
2.Which of the following have you noticed?
Dirt on your shirt (Score B)
Lost your sea legs (Score C)
Big red blotches under the skin (Score A)
Dirt on your shirt (Score B)
Lost your sea legs (Score C)
Big red blotches under the skin (Score A)
3.Which of the following do you frel when you wake in the morning?
Extremely fed-up (Score C)
Extremely fed-up (Score C)
Extremely poorly (Score A)
Extremely hungry(Score B)
How to Discover if you have Scury:
Mostly C's: You are a frustrated pirate who has spent to much time on dry land.
Cure: Get to sea.
Mostly B's: You are a starving pirate who has run out of clean clothes.
Cure: Make some filling ships biscuits and eat them while soaking in the bathtub.
Entirely A's: You are a sickly pirate, and probably have scurvy. If you develop these symptoms on land, put on your sea boots and get to the doc quick.
Cure: Stop at the next port for some oranges and green beans. Indulge.
Friday, December 21, 2007
Thursday, December 20, 2007
Wednesday, December 19, 2007
Tuesday, December 18, 2007
THE SUN AZIMUTH SIGHT
At times between sunrise and sunset, the gyro error may be abtained by computing the true azimuth of the sun by means of tables designed for azimuth computations or the use of the Sight Reduction Tables No. 229. I generally prefer PUB 229 for obtaining the true azimuth, inasmuch as these tables are widely available, and they are good for reducing a standard sight as well.
To use Tables No. 229 for computation of the exact azimuth of the sun, it is necessary to interpolate in the tables for the exact values of DR latitude, LHA, and declination of the sun for the time of the azimuth observations. The DR latitude is, obtained from the DR plot, while the LHA and declination of the sun are from the Nautical Almanac. I use a sight form designed for use in performing the triple interpolation in PUB 229.
Entering arguments for the tables for the exact azimuth determination are the whole degrees of DR latitude, LHA, and declination smaller than the exact calculated values. The tabulated azimuth angle Z corresponding with these integral arguments is first extracted and entered on the form as "Tab. Z". Next, the amount of change in the value of the tabulated azimuth angle, or "Z difference," is found between this tabulated azimuth angle and the value corresponding to the next higher integral degree of each of the three entering arguments. The Z difference for a one-degree increase of latitude, for example, is found by comparing the tabulated azimuth with the value contained in the next adjacent latitude column to the right, while keeping the LHA and declination the same. Likewise, the other two Z differences are found by successive comparisons of the tabulated azimuth angle for the next higher degrees of LHA and declination. All three Z differences are recorded on the azimuth form.
Using the exact minutes of each of the three entering arguments, each Z difference is then interpolated for the exact value of its corresponding argument; the result of each interpolation is the correction for the tabulated azimuth angle for the minutes of the argument. All three corrections are added to form the total correction to the tabulated azimuth angle. This total correction is then applied to obtain the exact azimuth angle corresponding to the exact values of the three entering arguments. As the final step, this exact azimuth angle is converted to a true azimuth, using the conversion formula printed on the azimuth sight form.
Comparison of the true azimuth computed with the observed azimuth of the sun yields the gyro error.
1. Obtain and record the DR latitude, exact LHA and declination of the sun for the time of the azimuth observation.
2. Using the integral degrees of these three quantities as entering arguments in Tables No. 229, extract the corresponding tabulated azimuth angle Z.
3. Obtain and record the three Z differences between this tabulated Z and the tabulated values for the next higher degree of each of the entering arguments.
4. Interpolate each Z difference for the correction corresponding to the exact minutes of its entering argument.
5. Add all three corrections to the form for total correction to tabulated Z.
6. Apply the correction to obtain the exact azimuth angle.
To use Tables No. 229 for computation of the exact azimuth of the sun, it is necessary to interpolate in the tables for the exact values of DR latitude, LHA, and declination of the sun for the time of the azimuth observations. The DR latitude is, obtained from the DR plot, while the LHA and declination of the sun are from the Nautical Almanac. I use a sight form designed for use in performing the triple interpolation in PUB 229.
Entering arguments for the tables for the exact azimuth determination are the whole degrees of DR latitude, LHA, and declination smaller than the exact calculated values. The tabulated azimuth angle Z corresponding with these integral arguments is first extracted and entered on the form as "Tab. Z". Next, the amount of change in the value of the tabulated azimuth angle, or "Z difference," is found between this tabulated azimuth angle and the value corresponding to the next higher integral degree of each of the three entering arguments. The Z difference for a one-degree increase of latitude, for example, is found by comparing the tabulated azimuth with the value contained in the next adjacent latitude column to the right, while keeping the LHA and declination the same. Likewise, the other two Z differences are found by successive comparisons of the tabulated azimuth angle for the next higher degrees of LHA and declination. All three Z differences are recorded on the azimuth form.
Using the exact minutes of each of the three entering arguments, each Z difference is then interpolated for the exact value of its corresponding argument; the result of each interpolation is the correction for the tabulated azimuth angle for the minutes of the argument. All three corrections are added to form the total correction to the tabulated azimuth angle. This total correction is then applied to obtain the exact azimuth angle corresponding to the exact values of the three entering arguments. As the final step, this exact azimuth angle is converted to a true azimuth, using the conversion formula printed on the azimuth sight form.
Comparison of the true azimuth computed with the observed azimuth of the sun yields the gyro error.
1. Obtain and record the DR latitude, exact LHA and declination of the sun for the time of the azimuth observation.
2. Using the integral degrees of these three quantities as entering arguments in Tables No. 229, extract the corresponding tabulated azimuth angle Z.
3. Obtain and record the three Z differences between this tabulated Z and the tabulated values for the next higher degree of each of the entering arguments.
4. Interpolate each Z difference for the correction corresponding to the exact minutes of its entering argument.
5. Add all three corrections to the form for total correction to tabulated Z.
6. Apply the correction to obtain the exact azimuth angle.
7. Convert the exact azimuth angle to a true azimuth.
8. Compare the true azimuth with the observed azimuth to get the gyro error.
AMPLITUDES
The celestial horizon differs from the one you see (the visible horizon) because it runs through the center of the earth. There are a lot of computations that must be done to determine the celestial horizon of a body, but for now we will just say that it is the horizon that a navigator uses for all celestial computations. When the center of the sun is on the celestial horizon, its lower limb (lower edge) is about two-thirds of the diameter of the sun above the visible horizon. When planets and stars are on the celestial horizon, they are a little more than one sun diameter above the visible horizon.
AMPLITUDE ON THE CELESTIAL HORIZON
The amplitude of a body can be taken directly from table 27 of Bowditch, volume II, if the body is observed when its center is on the celestial horizon. First of all, to observe the sun when it is on the celestial horizon, its lower limb should be about two-thirds of the diameter above the visible horizon, note the time and your compass bearing as observed by a bearing or azimuth circle to the sun. Next, with the Greenwich Mean Time (GMT) of your observation, you can use the right-hand daily pages of the Nautical Almanac to determine the sun's declination. From this known information, you can use table 27 of Bowditch to determine the amplitude.
AMPLITUDE ON THE CELESTIAL HORIZON
The amplitude of a body can be taken directly from table 27 of Bowditch, volume II, if the body is observed when its center is on the celestial horizon. First of all, to observe the sun when it is on the celestial horizon, its lower limb should be about two-thirds of the diameter above the visible horizon, note the time and your compass bearing as observed by a bearing or azimuth circle to the sun. Next, with the Greenwich Mean Time (GMT) of your observation, you can use the right-hand daily pages of the Nautical Almanac to determine the sun's declination. From this known information, you can use table 27 of Bowditch to determine the amplitude.
Monday, December 17, 2007
HOW TO DETERMINE YOUR LATITUDE BY POLARIS
STEP 1
Determine the Ho and LHA of Aries.
STEP 2
The Polaris Tables are entered to find the three parts of the Polaris correction. I use the sample problem at the bottom of the table on page 275 to provide the format and I add this to my sight form. The entering argument for the tables is the LHA of Aries.
STEP 3
To find the ao part, the table containing corrections for each integral degree of LHA is used with interpolation, if necessary, to arrive at the ao correction corresponding to LHA of Aries
STEP 4
Now we proceed to the middle table, in the same column, to obtain the a1 part. The entering argument in the left margin is the tabulated latitude closest to your DR latitude.
STEP 5
The a2 part is obtained from the lower third of the table, staying in the same column, opposite the month of observation. As a final step in finding the total correction to be applied to Ho, ao ,a1, and a2 are always added and the required - 1° is a constant.
STEP 6
Staying in the last column, enter the Azimuth part of the table with your DR latitude and extract the true azimuth of Polaris. Then find the gyro error and deviation. I have a form that I use, which I will publish on my blog with actual
Coast Guard exam questions.
Determine the Ho and LHA of Aries.
STEP 2
The Polaris Tables are entered to find the three parts of the Polaris correction. I use the sample problem at the bottom of the table on page 275 to provide the format and I add this to my sight form. The entering argument for the tables is the LHA of Aries.
STEP 3
To find the ao part, the table containing corrections for each integral degree of LHA is used with interpolation, if necessary, to arrive at the ao correction corresponding to LHA of Aries
STEP 4
Now we proceed to the middle table, in the same column, to obtain the a1 part. The entering argument in the left margin is the tabulated latitude closest to your DR latitude.
STEP 5
The a2 part is obtained from the lower third of the table, staying in the same column, opposite the month of observation. As a final step in finding the total correction to be applied to Ho, ao ,a1, and a2 are always added and the required - 1° is a constant.
STEP 6
Staying in the last column, enter the Azimuth part of the table with your DR latitude and extract the true azimuth of Polaris. Then find the gyro error and deviation. I have a form that I use, which I will publish on my blog with actual
Coast Guard exam questions.
LATITUDE AND AZIMUTH BY POLARIS
LATITUDE BY POLARIS
A second magnitude star called Polaris (north star) provides a reference for measurement in the northern hemisphere. Polaris has no counterpart in the Southern Hemisphere. Polaris may be located in the northern sky between the constellation Ursa Major (big dipper) and Cassiopeia. The two stars in the bowl of the dipper at the greatest distance from the handle, point toward the north star.
A second magnitude star called Polaris (north star) provides a reference for measurement in the northern hemisphere. Polaris has no counterpart in the Southern Hemisphere. Polaris may be located in the northern sky between the constellation Ursa Major (big dipper) and Cassiopeia. The two stars in the bowl of the dipper at the greatest distance from the handle, point toward the north star.
Polaris travels in a diurnal circle of small radius around the celestial north pole. This causes a special circumstance in celestial navigation. If you think of the four arguments in celestial navigation, Ho, Lat, Dec, and LHA. Polaris has some constants that make it a special case in celestial navigation. The declination of Polaris is 90 degree's North, and as long as your are in the northern hemisphere, your LHA will be 0 degree's. Therefore, there is only 2 arguments left - Ho and Latitude. For Polaris, the Nautical Almanac has a special (Polaris) table at the end of the white pages.
You can determine your latitude in the Northern Hemisphere by observing the Hs of Polaris, at a known time. From the time, and the DR or estimated longitude, compute the LHA of Aries. Correct Hs to Ho, and using the LHA Aries, approximate latitude, and date, determine corrections from Polaris tables ao, a1, and a2. Add total correction to Ho, and subtract 1 degree to obtain latitude. An good example is given in the table. The reason Polaris is so important is that you can determine your latitude and check your deviation
or gyro error of the compass.
POLARIS (NORTH STAR)
Polaris, more commonly known as The Pole Star, The North Star or simply North Star, is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole (42′ away as of 2006), making it the current northern Because α it lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole — the north celestial pole — Polaris stands almost motionless on the sky, and all the stars of the Northern sky appear to rotate around it. It makes an excellent fixed point from which to draw measurements for celestial navigation and for astrometry.
At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables or a rough rule of thumb.
Polaris will not always be the pole star. Other stars along this circle were the pole star in the past and will be again in the future, including Thuban and Vega. Polaris has been close to the actual position of the north pole for over 1000 years and during the course of the 21st century it will continue to approach the exact theoretical position, reaching its closest approach on 24 March 2100 (almost 0.45° away). Subsequently it will begin to pull away.
In the Northern Hemisphere, it is easy to find Polaris by following the line traced from Merak to Dubhe and Ursae Majoris, also known as the Pointers), the two stars at the end of the bowl of the Big Dipper(or Plough). One can also follow the central point of the "W" shape of Cassiopeia. Polaris is not visible from the Southern Hemisphere except from an elevated position near the equator.
Polaris's fame as the North Star has given rise to a persistent misconception that it is the brightest star in the sky. Although Polaris is a relatively bright star and is conspicuous since no other stars of similar brightness are close to it, it is nowhere near the brightest; it is actually the 48th brightest star in the night sky. The brightest star in the night sky is Sirius.There is no real southern pole star. The only star visible to the naked eye that is close to the south celestial pole is the dim Sigma Octantis, sometimes called Polaris Australis. However, the bright Southern Cross (Crux) points fairly accurately towards the south celestial pole.
At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables or a rough rule of thumb.
Polaris will not always be the pole star. Other stars along this circle were the pole star in the past and will be again in the future, including Thuban and Vega. Polaris has been close to the actual position of the north pole for over 1000 years and during the course of the 21st century it will continue to approach the exact theoretical position, reaching its closest approach on 24 March 2100 (almost 0.45° away). Subsequently it will begin to pull away.
In the Northern Hemisphere, it is easy to find Polaris by following the line traced from Merak to Dubhe and Ursae Majoris, also known as the Pointers), the two stars at the end of the bowl of the Big Dipper(or Plough). One can also follow the central point of the "W" shape of Cassiopeia. Polaris is not visible from the Southern Hemisphere except from an elevated position near the equator.
Polaris's fame as the North Star has given rise to a persistent misconception that it is the brightest star in the sky. Although Polaris is a relatively bright star and is conspicuous since no other stars of similar brightness are close to it, it is nowhere near the brightest; it is actually the 48th brightest star in the night sky. The brightest star in the night sky is Sirius.There is no real southern pole star. The only star visible to the naked eye that is close to the south celestial pole is the dim Sigma Octantis, sometimes called Polaris Australis. However, the bright Southern Cross (Crux) points fairly accurately towards the south celestial pole.
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