The main tool used by navigators and pilots on board a Chinese ship was the water compass. For keeping time, incense, which was graded to burn a certain amount in a certain time, was used. As on later western vessels, a day was broken into watches. Unlike those western vessels, the day was broken into ten watches of 2.4 hours each.
During the time of Zheng He and the Treasure Fleet, the average ship could travel about twenty miles per watch, at a speed of about eight knots. Speed was determined by throwing an object over the bow of the ship, walking the length of the ship while watching the object, and measuring, by chanting a rhyme, how long it took for the vessel to pass the object.
Latitude was found using a similar theory, though slightly different method, than the European crossstaff. Navigators measured the altitude of Polaris or the Southern Cross above the horizon with an instrument called a qianxingban. The qianxingban was a board consisting of twelve pieces of square wood, the board would be aligned with the horizon, and navigators used the lengths of their arms to calculate the position of the stars. Another, simpler instrument used for this purpose was the liangtianchi, a vertical ruler.
Captains also used sailing charts, which were much larger than their western counterparts. It was unrolled in sections, depending on where the ship was. The charts used by the Treasure Fleet were a series of sailing directions in the form of compass bearings and lengths of watches from port to port, across the Indian Ocean. It also showed any landmarks that might help the captains recognize their location. In addition to the sailing chart, star maps were used.
To find depth and to determine what was on the bottom of the body of water traveled, pilots used a lead line. These were very similar to the lead lines used by western sailors.
Thursday, April 30, 2009
Friday, April 24, 2009
Whats the Difference Between Dolphins and Porpoises
How do you tell the difference between dolphins and porpoises ?
For years many people have used the word dolphin and porpoise interchangeably. Many people believe that a dolphin is a porpoise and vice versa. They are very similar and have many common characteristics. But, there are some differences between the two.
Both dolphins and porpoises are mammals. Even though they both live in the ocean, they are not fish. They do not have breathing gills. They have lungs and breathe air. Both also give birth to live young and nurse those young. A mammal is also characterized by having the presence of hair: when dolphins and porpoises are small, there are small hair follicles.
Dolphins and porpoises belong to the same scientific order, Cetacea. This order includes all whales, even the great whales, to which both dolphins and porpoises are related. All cetaceans are completely aquatic mammals, have a streamlined body, a tail fluke, and a blowhole (which is what these air-breathing animals use to breathe). The dolphins and porpoises are also classified in the scientific suborder, Odontoceti, which are the toothed whales. All odontocetes also have the ability to echolocate, the ability to detect objects in their underwater environment using the echoes of a sound, much like sonar.
Porpoises and dolphins are classified into two different families. The porpoises are in the family Phocoenidae and the dolphins are in the family Delphinidae. When separated at the family level, dolphins and porpoises are as physically different as cats and dogs.
In comparison to dolphins, porpoises are very small. Porpoises seldom exceed 7 feet in length, whereas many dolphins can exceed 10 feet in length. Porpoises are also more robust than dolphins. Dolphins have a lean sleek body, whereas porpoises often appear chubby. The dorsal fin (the fin on the back of the animal) in porpoises is also triangular, looking more like a shark. The dorsal fin of the dolphin is shaped in a wave. Porpoises lack a rostrum or a beak. This rostrum is very prominent in dolphins. The teeth of the porpoise are spade-shaped, whereas the teeth of the dolphin are conical or cone-shaped.
Many porpoises do not live past their mid-teens. Porpoises have an intensive reproduction schedule that may play a role in their lack of longevity. A porpoise can become pregnant each year, give birth, and then it can become pregnant again five or six weeks later, so it can be nursing and pregnant at the same time. This can also happen in dolphins, but dolphins are larger in size and it seems their body is suited for handling such occurrences, and anyhow it is less common. Dolphins can live in an upwards of fifty years.
There are many behavior differences as well as physical differences. For the most part, porpoises are shy animals. They do not approach people or boats. The dolphin, on the other hand, if often seen riding the bow wave of fishing boats. You rarely see porpoises at the surface unless they are coming up for a breath.
The dolphin, rather than the porpoise, is often seen in marine animal shows. This comes back to the idea that dolphins tend to show a lesser fear of man than porpoises. This is why dolphins, not porpoises, get stuck in tuna nets. For this reason dolphins are widely studied whereas porpoises are not.
Both dolphins and porpoises have an unique social order. While both use their teeth as a form of tactile communication within the social group, scientists believe that unlike dolphins, porpoises do not use underwater whistles to communicate. Dolphins actually use their blowholes to create a whistling noise, which is used particularly often to communicate between mother and calf.
Dolphins and porpoises have a lot in common. There are some differences, but the similarities among their behavior and looks outweigh the differences. You are more likely to see a dolphin, both in the wild and captivity. Consider yourself lucky if you encounter a porpoise in either situation.
Saturday, April 18, 2009
Fire Aboard The SS African Star
On the morning of March 16, 1968, at about 0340, the dry-cargo vessel SS African Star collided in a meeting situation with the tank barge Intercity no. 11 in the lower Mississippi River, in the vicinity of mile 46 Above Head of Passes (AHP). The African Star's bow penetrated the Intercity no. 11 on the after port side, at an angle of 45°. The motor towing vessel Midwest Cities was pushing two tank barges, Intercity no. 11 and Intercity no. 14 (the forward barge). The two tank barges were identical. A few minutes before the collision, the African Star was making about 16 knots on a 140° true course, the Midwest Cities was making 6 knots on a 320° true course with a relative closing speed of 22 knots. Visibility was good and each vessel had been advised of the other vessel's movements on its own radio frequency. Because of the lack of a common radiotelephone frequency, direct communication between the vessels was not possible.
Both vessels were equipped with marine radar units. Both units were in operation prior to and at the time of the casualty, but neither unit was being continuously observed by watch personnel. The pilot of each vessel sighted the navigation lights of the other vessel 1 1/2 miles, and later sighted the other vessel on radar. Witnesses in passing vessels reported that they could easily see the navigation lights on the Midwest Cities, Intercity no. 14 and African Star. The movements of the vessels were not materially affected by wind or current. The steering gear and machinery of both vessels were in good operating order. The African Star had a licensed pilot, but the Midwest Cities had an unlicensed pilot, however, both pilots had extensive experience on the Mississippi River. There was a lookout on the bow of the African Star, but none on the Midwest Cities. The master, third mate and helmsman were also on the bridge of the African Star.
The Collision
Different versions of the maneuvers were given by personnel on each of the two vessels.
Midwest Cities Version - The Midwest Cities was running parallel to the side of the river, about 250 feet from the east bank. The pilot considered it to be a head-and-head meeting situation, and the pilot sounded the appropriate one-blast whistle signal for a port-to-port passing. The African Star responded with one blast. He assumed a safe passage until the African Star sounded two blast when her bow was abeam the lead barge. He saw the African Star's green sidelight and responded with one blast. He then blew four blasts on the whistle, backed full astern from full ahead and put the rudder hard right. However, it was too late to avert a collision between the African Star and barge Intercity no. 11.
African Star Version - The pilot of the African Star stated that his vessel was slightly west of mid river when he sighted the Midwest Cities two white tow lights and green sidelights on his starboard bow. The tow appeared to be favoring the west bank and running parallel to it. It appeared to him to be a normal starboard-to-starboard meeting situation, not a head-and-head meeting. When the Midwest Cities tow was 1/2 to 3/4 mile ahead, he sounded two short blasts on the whistle, but no reply was heard. As the pilot headed for the radar, the third mate called his attention to the tow crossing his starboard bow showing red sidelights. This was about 2 minutes after the two-blast signal was sounded. Hard right rudder, one blast and then emergency full astern were ordered and executed. By this time, the situation was beyond the point of corrective action a collision was unavoidable. Full astern was in effect a minute before the collision.
In his analysis of the incident, the commandant of the U.S. Coast Guard concluded that the witnesses gave such conflicting testimony that it was impossible to reconstruct the events leading up to the collision.
The Fire
Intercity no. 11 was loaded to a draft of about 9 feet 6 inches, corresponding to approximately 19,000 barrels of crude oil. An analysis of the Louisiana "sweet" crude it carried revealed a 30.6° API a flash point (Pensky Martens) of 80.0°F, and a Reid vapor pressure of 3.2 psia, which categorized the product as a grade C flammable liquid. When the collision occurred, the general alarm was sounded on the order of the master of the African Star. At this time, the oncoming watch personnel were in varying degrees of readiness and, except for those on watch, all crew members and passengers were asleep or resting in their quarters.
In less than a minute, fire broke out and several explosions occurred. The most likely source of ignition was high heat due to metal-to-metal friction or sparks, produced when the barge was sheared by the bow of the African Star. Another possible source of ignition was sparks generated by the severing of the electrical cable leading to the navigation lights on Intercity no. 14. When fire broke out on the barge and in the surrounding water, the pilot of the Midwest Cities backed full to break the port wire and to clear the intense fire. He estimated it took about a minute to get free; his vessel was backing toward the west bank. Intercity no. 11 grounded and sank near the west bank at mile 45.7 (AHP). The Midwest Cities was downwind of the point of collision and escaped with only minor damage.
The southeasterly wind carried flammable vapors over the African Star from bow to stem (because of the vessel's position relative to the wind direction). The flammable vapors ignited, engulfing the vessel in flames. The pilot backed clear and intentionally grounded the vessel on the west bank at mile 45.8 (AHP). The tarpaulins had been ignited, and there were fires in holds 2, 4 and 5. Containers and other deck cargo were burning, as was the paint on the ship. Dense smoke filled the engine room and accommodation spaces.
Firefighting and Rescue
Problems were encountered in lowering the lifeboat and launching the inflatable life raft; the boat cover and man ropes had burned, and the plastic cover of the life raft had ignited. The intense fire, heat and smoke in the quarters gutted the passageways, and a number of passengers and crew members were trapped. Several people tried to escape through portholes when they found that the passageways outside their quarters were impassable. Others were burned when their life preservers and clothing ignited. For a while the fire and heat on the port side were too intense to endure. There was some minor confusion during the first few minutes after the alarm was sounded. However, this was quickly dispelled under the leadership of the master and his officers. After the African Star was grounded, the master went to the cabin deck to see to the safety of the passengers and crew.
During this time, he became seriously burned about the feet, face and hands. As a result, he was immobilized and had to be carried back to the bridge by the crew. At first, burning oil on the water surrounding the vessel prevented personnel from jumping overboard to get away from the burning vessel. The second mate gathered a number of passengers and crew into a small room on the African Star for refuge until the fire subsided. He then supervised the extinguishment of small fires in and around no. 1 lifeboat. By this time, the current and the movement of the African Star had separated the vessel from the oil burning on the water, the lifeboat was lowered to the edge of the deck and the injured crew members and passengers were assisted into the boat and lowered to the water's edge.
Other crewmen and passengers were able to climb or jump into the water and swim ashore. The second mate observed large fires burning aft on the main deck. He organized a firefighting team that advanced hoselines to the area. They were successful in confining the deck fires and cooling the flammable-liquid cargo. An oiler in the engine room was forced to leave because of difficulty breathing in the smoke. However, the chief engineer, third assistant engineer, and fireman / watertender continued to maintain the engine room plant in full operation. Power was maintained to keep the vessel aground, the lights on and the fire and bilge pumps in operation.
Rescue operations had commenced following the Midwest Cities request for immediate assistance via the marine operator in New Orleans. Badly burned victims were quickly evacuated by U.S. Coast Guard helicopters. This operation is credited with saving the lives of a number of people injured on the African Star. The Midwest Cities, a New Orleans fireboat and a local ferry with fire apparatus on board assisted Coast Guard boats in fighting the fire. Firefighting was complicated by inaccessibility to the cargo manifest of hazardous materials located in the chief mate's room. In addition, a number of deck fire hoses had been burned. The combustibles on deck and in the holds continued to burn after the vapor and oil spray fires had subsided.
Firefighting by the African Star crew controlled the fire until the U.S. Coast Guard vessels and other help arrived. The fire in hold no. 5 was contained by use of the ship's C02 extinguishing system. At about 0530, the fires on board the African Star had been brought under control, and the Midwest Cities departed to retrieve Intercity no. 14, adrift in the river. Intercity no. 14 was undamaged. The many fatalities and injuries sustained on board the African Star were due to the rapid spread of fire, the heat and smoke in living spaces and the burning oil on the water surrounding the vessel, which kept most personnel from immediately jumping overboard. A total of 11 passager and 52 crewmen on the freighter, 2 passengers were killed and 9 were injured, 15 crew members were killed, 4 were missing and presumed dead, 31 were injured, and 2 escaped injury. Many more lives would have been lost, but for the gallant efforts and bravery of African Star crewmen and others involved in the rescue and firefighting operations. A collision and fire of this magnitude must point up both weaknesses (areas where seamen can learn from the mistakes of others) and strengths (examples of leadership, teamwork and heroism). Some of the more important lessons to be learned include the following:
Whistle signals are not of themselves a reliable means of communicating a vessel's passing or turning intentions. Bridge-to bridge radiotelephone communication on a single frequency would probably have prevented this tragedy. It is now required by law. Uncertainties and difficulties are experienced in applying the inland rules of the road to arrange a safe passing. Passing requires the use of visual and verbal communication in both directions, plus good judgment. A properly equipped vessel can with stand a serious collision and fire. A disciplined and well-trained crew can keep the vessel afloat, maintain control of the wheelhouse and engine room and successfully combat the lire. Leadership, courage and discipline are essential traits for officers and crewmen in the merchant marine. The value of these traits becomes most evident in an emergency situation such as a serious fire.
Both vessels were equipped with marine radar units. Both units were in operation prior to and at the time of the casualty, but neither unit was being continuously observed by watch personnel. The pilot of each vessel sighted the navigation lights of the other vessel 1 1/2 miles, and later sighted the other vessel on radar. Witnesses in passing vessels reported that they could easily see the navigation lights on the Midwest Cities, Intercity no. 14 and African Star. The movements of the vessels were not materially affected by wind or current. The steering gear and machinery of both vessels were in good operating order. The African Star had a licensed pilot, but the Midwest Cities had an unlicensed pilot, however, both pilots had extensive experience on the Mississippi River. There was a lookout on the bow of the African Star, but none on the Midwest Cities. The master, third mate and helmsman were also on the bridge of the African Star.
The Collision
Different versions of the maneuvers were given by personnel on each of the two vessels.
Midwest Cities Version - The Midwest Cities was running parallel to the side of the river, about 250 feet from the east bank. The pilot considered it to be a head-and-head meeting situation, and the pilot sounded the appropriate one-blast whistle signal for a port-to-port passing. The African Star responded with one blast. He assumed a safe passage until the African Star sounded two blast when her bow was abeam the lead barge. He saw the African Star's green sidelight and responded with one blast. He then blew four blasts on the whistle, backed full astern from full ahead and put the rudder hard right. However, it was too late to avert a collision between the African Star and barge Intercity no. 11.
African Star Version - The pilot of the African Star stated that his vessel was slightly west of mid river when he sighted the Midwest Cities two white tow lights and green sidelights on his starboard bow. The tow appeared to be favoring the west bank and running parallel to it. It appeared to him to be a normal starboard-to-starboard meeting situation, not a head-and-head meeting. When the Midwest Cities tow was 1/2 to 3/4 mile ahead, he sounded two short blasts on the whistle, but no reply was heard. As the pilot headed for the radar, the third mate called his attention to the tow crossing his starboard bow showing red sidelights. This was about 2 minutes after the two-blast signal was sounded. Hard right rudder, one blast and then emergency full astern were ordered and executed. By this time, the situation was beyond the point of corrective action a collision was unavoidable. Full astern was in effect a minute before the collision.
In his analysis of the incident, the commandant of the U.S. Coast Guard concluded that the witnesses gave such conflicting testimony that it was impossible to reconstruct the events leading up to the collision.
The Fire
Intercity no. 11 was loaded to a draft of about 9 feet 6 inches, corresponding to approximately 19,000 barrels of crude oil. An analysis of the Louisiana "sweet" crude it carried revealed a 30.6° API a flash point (Pensky Martens) of 80.0°F, and a Reid vapor pressure of 3.2 psia, which categorized the product as a grade C flammable liquid. When the collision occurred, the general alarm was sounded on the order of the master of the African Star. At this time, the oncoming watch personnel were in varying degrees of readiness and, except for those on watch, all crew members and passengers were asleep or resting in their quarters.
In less than a minute, fire broke out and several explosions occurred. The most likely source of ignition was high heat due to metal-to-metal friction or sparks, produced when the barge was sheared by the bow of the African Star. Another possible source of ignition was sparks generated by the severing of the electrical cable leading to the navigation lights on Intercity no. 14. When fire broke out on the barge and in the surrounding water, the pilot of the Midwest Cities backed full to break the port wire and to clear the intense fire. He estimated it took about a minute to get free; his vessel was backing toward the west bank. Intercity no. 11 grounded and sank near the west bank at mile 45.7 (AHP). The Midwest Cities was downwind of the point of collision and escaped with only minor damage.
The southeasterly wind carried flammable vapors over the African Star from bow to stem (because of the vessel's position relative to the wind direction). The flammable vapors ignited, engulfing the vessel in flames. The pilot backed clear and intentionally grounded the vessel on the west bank at mile 45.8 (AHP). The tarpaulins had been ignited, and there were fires in holds 2, 4 and 5. Containers and other deck cargo were burning, as was the paint on the ship. Dense smoke filled the engine room and accommodation spaces.
Firefighting and Rescue
Problems were encountered in lowering the lifeboat and launching the inflatable life raft; the boat cover and man ropes had burned, and the plastic cover of the life raft had ignited. The intense fire, heat and smoke in the quarters gutted the passageways, and a number of passengers and crew members were trapped. Several people tried to escape through portholes when they found that the passageways outside their quarters were impassable. Others were burned when their life preservers and clothing ignited. For a while the fire and heat on the port side were too intense to endure. There was some minor confusion during the first few minutes after the alarm was sounded. However, this was quickly dispelled under the leadership of the master and his officers. After the African Star was grounded, the master went to the cabin deck to see to the safety of the passengers and crew.
During this time, he became seriously burned about the feet, face and hands. As a result, he was immobilized and had to be carried back to the bridge by the crew. At first, burning oil on the water surrounding the vessel prevented personnel from jumping overboard to get away from the burning vessel. The second mate gathered a number of passengers and crew into a small room on the African Star for refuge until the fire subsided. He then supervised the extinguishment of small fires in and around no. 1 lifeboat. By this time, the current and the movement of the African Star had separated the vessel from the oil burning on the water, the lifeboat was lowered to the edge of the deck and the injured crew members and passengers were assisted into the boat and lowered to the water's edge.
Other crewmen and passengers were able to climb or jump into the water and swim ashore. The second mate observed large fires burning aft on the main deck. He organized a firefighting team that advanced hoselines to the area. They were successful in confining the deck fires and cooling the flammable-liquid cargo. An oiler in the engine room was forced to leave because of difficulty breathing in the smoke. However, the chief engineer, third assistant engineer, and fireman / watertender continued to maintain the engine room plant in full operation. Power was maintained to keep the vessel aground, the lights on and the fire and bilge pumps in operation.
Rescue operations had commenced following the Midwest Cities request for immediate assistance via the marine operator in New Orleans. Badly burned victims were quickly evacuated by U.S. Coast Guard helicopters. This operation is credited with saving the lives of a number of people injured on the African Star. The Midwest Cities, a New Orleans fireboat and a local ferry with fire apparatus on board assisted Coast Guard boats in fighting the fire. Firefighting was complicated by inaccessibility to the cargo manifest of hazardous materials located in the chief mate's room. In addition, a number of deck fire hoses had been burned. The combustibles on deck and in the holds continued to burn after the vapor and oil spray fires had subsided.
Firefighting by the African Star crew controlled the fire until the U.S. Coast Guard vessels and other help arrived. The fire in hold no. 5 was contained by use of the ship's C02 extinguishing system. At about 0530, the fires on board the African Star had been brought under control, and the Midwest Cities departed to retrieve Intercity no. 14, adrift in the river. Intercity no. 14 was undamaged. The many fatalities and injuries sustained on board the African Star were due to the rapid spread of fire, the heat and smoke in living spaces and the burning oil on the water surrounding the vessel, which kept most personnel from immediately jumping overboard. A total of 11 passager and 52 crewmen on the freighter, 2 passengers were killed and 9 were injured, 15 crew members were killed, 4 were missing and presumed dead, 31 were injured, and 2 escaped injury. Many more lives would have been lost, but for the gallant efforts and bravery of African Star crewmen and others involved in the rescue and firefighting operations. A collision and fire of this magnitude must point up both weaknesses (areas where seamen can learn from the mistakes of others) and strengths (examples of leadership, teamwork and heroism). Some of the more important lessons to be learned include the following:
Whistle signals are not of themselves a reliable means of communicating a vessel's passing or turning intentions. Bridge-to bridge radiotelephone communication on a single frequency would probably have prevented this tragedy. It is now required by law. Uncertainties and difficulties are experienced in applying the inland rules of the road to arrange a safe passing. Passing requires the use of visual and verbal communication in both directions, plus good judgment. A properly equipped vessel can with stand a serious collision and fire. A disciplined and well-trained crew can keep the vessel afloat, maintain control of the wheelhouse and engine room and successfully combat the lire. Leadership, courage and discipline are essential traits for officers and crewmen in the merchant marine. The value of these traits becomes most evident in an emergency situation such as a serious fire.
Thursday, April 16, 2009
Viking Navigation
Viking navigation was handled by specially trained men who used the stars and sun to aid in their voyages. While birds were brought along on some voyages to be let go in order to follow them to the nearest land, they relied on navigation tools called peloruses, the sun stone, the bearing dial, the sun shadow board, and the sun compass.
Primarily, the Vikings used the North Star or Polaris to help them navigate at night. Located in the night sky, directly over the north celestial pole, the distance from the North Star to the horizon was compared to the height of the star when they were at home. This measurement helped them to determine their latitude.
The pelorus, similar to the ones used today, was an instrument much like a mariner's compass, but without magnetic needles. It had two sight vanes and was mounted in such a way that the bearings of objects could be observed. The bearing dial or bearing circle was used to determine the latitude of the sun. It had a small platform with a vertical pin in the middle and a pointer, and it was used to track the position of the sun in the sky by marking the placement of shadows on the platform.
The sun shadow board was used at noon to double check whether the ship was on the correct course or not. It was placed in a bowl of water to keep it level and the gnomen, the pin in the center of the board, marked the shadow of the noon sun. Circles marked on the board gave the sailors an area in which they could travel and still remain within their desired latitude. If the shadow of the noon sun extended beyond the circle, they knew they had traveled too far north. If the shadow was inside the line, they were too far south. The sun shadow board's midday measurement helped the Vikings make course corrections each noon, but was of know use to the sailors in cloudy or foggy weather.
The sun stone was used on the days when fog or clouds obscured the sun. The stone, a mineral called Icelandic spar, would change color as it was turned in the light. A certain color would indicate the position of the sun through the fog or clouds but could only be used when there was at least a sliver of blue sky. The semi-wheel was a chart that recorded the year round observations of the sun and measurements of its positions. The Vikings of Iceland collected this data, also noting where on the horizon the sun rose and set each day. This collection of information was used to determine their latitude as well as the cardinal directions, North, South, East and West.
The sun compass recorded the different paths the sun took across the sky during the different seasons of the year. By drawing these different hyperbolas on the face of the sun compass, the Vikings had a record of the sun's position for each time of year. They were able to determine, with great accuracy and at any time of day, their position at sea by rotating the disk of the sun compass until the shadow of the tip in the center of the compass fell on the hyperbola for that time of year.
Primarily, the Vikings used the North Star or Polaris to help them navigate at night. Located in the night sky, directly over the north celestial pole, the distance from the North Star to the horizon was compared to the height of the star when they were at home. This measurement helped them to determine their latitude.
The pelorus, similar to the ones used today, was an instrument much like a mariner's compass, but without magnetic needles. It had two sight vanes and was mounted in such a way that the bearings of objects could be observed. The bearing dial or bearing circle was used to determine the latitude of the sun. It had a small platform with a vertical pin in the middle and a pointer, and it was used to track the position of the sun in the sky by marking the placement of shadows on the platform.
The sun shadow board was used at noon to double check whether the ship was on the correct course or not. It was placed in a bowl of water to keep it level and the gnomen, the pin in the center of the board, marked the shadow of the noon sun. Circles marked on the board gave the sailors an area in which they could travel and still remain within their desired latitude. If the shadow of the noon sun extended beyond the circle, they knew they had traveled too far north. If the shadow was inside the line, they were too far south. The sun shadow board's midday measurement helped the Vikings make course corrections each noon, but was of know use to the sailors in cloudy or foggy weather.
The sun stone was used on the days when fog or clouds obscured the sun. The stone, a mineral called Icelandic spar, would change color as it was turned in the light. A certain color would indicate the position of the sun through the fog or clouds but could only be used when there was at least a sliver of blue sky. The semi-wheel was a chart that recorded the year round observations of the sun and measurements of its positions. The Vikings of Iceland collected this data, also noting where on the horizon the sun rose and set each day. This collection of information was used to determine their latitude as well as the cardinal directions, North, South, East and West.
The sun compass recorded the different paths the sun took across the sky during the different seasons of the year. By drawing these different hyperbolas on the face of the sun compass, the Vikings had a record of the sun's position for each time of year. They were able to determine, with great accuracy and at any time of day, their position at sea by rotating the disk of the sun compass until the shadow of the tip in the center of the compass fell on the hyperbola for that time of year.
Tuesday, April 14, 2009
Polynesian Navigation
As Captain James Cook was conducting his voyages of exploration and discovery, Polynesian navigators had already successfully explored and settled the islands from New Zealand to Hawaii. Remarkably, the Polynesians had developed a sophisticated and reliable means of wayfinding based not on science and mathematics, but rather on their innate knowledge of the seas and sky.
By using the sun, stars, sea swell patterns, cloud formations, and seamarks such as bird flight habits, Polynesian navigators were able to steer their canoes over distances that amazed European navigators including the two thousand miles between Tahiti and the Hawaiian Islands. The Polynesian star compass was the key to finding direction at sea. The four cardinal points (north, south, east, and west) were located according to the rising and setting sun. During night voyaging, stars formed reference points. Polynesian navigators memorized the star compass as well as knmvn islands whose locations corresponded to points of the compass.
In training a navigator would name an island as the center point, then go around the compass points naming the islands that lay in each direction. Beyond navigating by the sun and stars, the Polynesians used their extensive knowledge of the sea to successfully guide them through their voyages. By careful observation of sea swell patterns, wind direction, cloud formations, and patterns of bird flight and flotsam, traditional Pacific navigators pieced together the course they chose to follow.
Cloud Formations:
As clouds moved over sea and land, the Polynesians noted that clouds tend to be drawn to land in distinctive "V" formations. This cloud pattern is created by the reflection of heat radiated from the island. Many navigators also noted slight color changes in clouds over land, and were able to distinguish the landform from the color, a slight green indicated lagoon islands, bright clouds indicated sand, and dark clouds markrd forested areas.
Flight of birds:
Flight patterns of specific species provided a reliable means of determining the direction of land. The fairy and noddy terns were especially important, as both species nest on land, and neither swims. Both terns fly to sea in the morning and return to land at dusk. By observing the habits of these birds. Polynesian navigators could not only determine the direction of land, but also its approximate distance. Fairy terns have a flight range of about one hundred twenty miles, while noddy terns have a range of about 40 miles.
Flotsam:
Floating debris such as palm fronds, coconuts, and other vegetation also signaled nearby land. Experimental "wayfinding" has been traditionally performed in Micronesia, and is regaining credence as an art in Polynesia.
By using the sun, stars, sea swell patterns, cloud formations, and seamarks such as bird flight habits, Polynesian navigators were able to steer their canoes over distances that amazed European navigators including the two thousand miles between Tahiti and the Hawaiian Islands. The Polynesian star compass was the key to finding direction at sea. The four cardinal points (north, south, east, and west) were located according to the rising and setting sun. During night voyaging, stars formed reference points. Polynesian navigators memorized the star compass as well as knmvn islands whose locations corresponded to points of the compass.
In training a navigator would name an island as the center point, then go around the compass points naming the islands that lay in each direction. Beyond navigating by the sun and stars, the Polynesians used their extensive knowledge of the sea to successfully guide them through their voyages. By careful observation of sea swell patterns, wind direction, cloud formations, and patterns of bird flight and flotsam, traditional Pacific navigators pieced together the course they chose to follow.
Cloud Formations:
As clouds moved over sea and land, the Polynesians noted that clouds tend to be drawn to land in distinctive "V" formations. This cloud pattern is created by the reflection of heat radiated from the island. Many navigators also noted slight color changes in clouds over land, and were able to distinguish the landform from the color, a slight green indicated lagoon islands, bright clouds indicated sand, and dark clouds markrd forested areas.
Flight of birds:
Flight patterns of specific species provided a reliable means of determining the direction of land. The fairy and noddy terns were especially important, as both species nest on land, and neither swims. Both terns fly to sea in the morning and return to land at dusk. By observing the habits of these birds. Polynesian navigators could not only determine the direction of land, but also its approximate distance. Fairy terns have a flight range of about one hundred twenty miles, while noddy terns have a range of about 40 miles.
Flotsam:
Floating debris such as palm fronds, coconuts, and other vegetation also signaled nearby land. Experimental "wayfinding" has been traditionally performed in Micronesia, and is regaining credence as an art in Polynesia.
Saturday, April 11, 2009
Timekeeping and Longitude
One of the most pressing problems of navigation during Captain James Cook's time was the inability to accurately calculate longitude. For many years, sailors could find their latitude with the use of celestial navigation. An instrument called a quadrant was used to sight a particular star or the sun, and then the angle between the horizon, the star, were measured. Latitude could then be calculated from this measurement.
Longitude was another problem altogether, to be able to calculate longitude, one needed to know exactly what date and time it was to complete the celestial calculation. In 1714, the Longitude Act was passed in Britain. There were monetary prizes of 20,000 pounds for a method to determine longitude to an accuracy of half a degree of a great circle, 15,000 pounds for a method accurate to within two-thirds of a degree, and 10,000 pounds for a method accurate to within one degree. One "degree" would be about 60 miles.
There were two major schools of thought on the subject of longitude: those who believed that accurate star and lunar charts alone could lead navigators to an accurate measurement of longitude, and those who believed that an accurate timekeeping device would suffice for the same reason. Each had problems, the celestial option had the issues that on cloudy days celestial navigation was troublesome and that one had to be an able mathematician to perform the necessary calculations, and the timekeeper's problem was that no clock yet created was nearly accurate enough to keep correct time over long periods, nor during varying levels of temperature and humidity.
The main figures representing each side of this argument were Nevil Maskelyne on the side of celestial navigation, and John Harrison on the side of the clockmakers. One problem for Harrison was that Maskelyne had a very important position: He was Astronomer Royal. Captain Cook and his scientists carried out tests for the Board of Longitude, comparing the lunar distance method with chronometers modeled after Harrison's designs. He took a chronometer named K-l (Larcum Kendall's first, and a close copy of H-4, Harrison's fourth attempt), and three copies of it by a clockmaker named John Arnold. Cook's log refers to K-l on manyoccasions, calling it "our trusty friend the watch," and noting "It would not be doing justice to Mr. Harrison and Mr. Kendall if I did not own that we have received very great assistance from this useful and valuable timepiece." Cook believed so much in the use of the chronometer that he carried it on his third voyage as well.
The lunar distance method, which an able navigator such as Cook could master, worked effectively, but only under clear skies. Cook preferred the chronometer, however, since Nevil Maskelyne was on the Board of Longitude, Harrison did not receive the full award until the king intervened for him.
Longitude was another problem altogether, to be able to calculate longitude, one needed to know exactly what date and time it was to complete the celestial calculation. In 1714, the Longitude Act was passed in Britain. There were monetary prizes of 20,000 pounds for a method to determine longitude to an accuracy of half a degree of a great circle, 15,000 pounds for a method accurate to within two-thirds of a degree, and 10,000 pounds for a method accurate to within one degree. One "degree" would be about 60 miles.
There were two major schools of thought on the subject of longitude: those who believed that accurate star and lunar charts alone could lead navigators to an accurate measurement of longitude, and those who believed that an accurate timekeeping device would suffice for the same reason. Each had problems, the celestial option had the issues that on cloudy days celestial navigation was troublesome and that one had to be an able mathematician to perform the necessary calculations, and the timekeeper's problem was that no clock yet created was nearly accurate enough to keep correct time over long periods, nor during varying levels of temperature and humidity.
The main figures representing each side of this argument were Nevil Maskelyne on the side of celestial navigation, and John Harrison on the side of the clockmakers. One problem for Harrison was that Maskelyne had a very important position: He was Astronomer Royal. Captain Cook and his scientists carried out tests for the Board of Longitude, comparing the lunar distance method with chronometers modeled after Harrison's designs. He took a chronometer named K-l (Larcum Kendall's first, and a close copy of H-4, Harrison's fourth attempt), and three copies of it by a clockmaker named John Arnold. Cook's log refers to K-l on manyoccasions, calling it "our trusty friend the watch," and noting "It would not be doing justice to Mr. Harrison and Mr. Kendall if I did not own that we have received very great assistance from this useful and valuable timepiece." Cook believed so much in the use of the chronometer that he carried it on his third voyage as well.
The lunar distance method, which an able navigator such as Cook could master, worked effectively, but only under clear skies. Cook preferred the chronometer, however, since Nevil Maskelyne was on the Board of Longitude, Harrison did not receive the full award until the king intervened for him.
Thursday, April 9, 2009
Astrolade a Navigational Tool
One of the oldest of all the altitiude measuring devices, the Astrolabe is a angle measuring tool thats name comes from the Greek, " to take a star" It was invented by the greek astronomer and mathematician, Hipparchus, it was used for astronomy and astrology.
As an astronomer's tool, the Astrolabe was introduced to the Europeans by Arab astronomers in the 10th century. But the first documented use of it used at sea is in 1481 on a voyage down the African coast by Portuguese explorers. It is likely that it was in use by sailors for many years before that.
So how does it work? To correctly measure the angle of the sun or a star, the Astrolabe must hang down so that it is perpendicular to the ocean. If it's tilted right or left, or front to back, the angle will not be accurate. To keep it straight, the user holds it with a finger through a ring and lets the Astrolabe dangle. There are two plates on a rotating arm. They each have a pinhole perfectly lined up so that when the sun shines through the top one, and hits the second pinhole, the angle is accurate. You then read from the scale along the circumference.
For Polaris, you can sight over the edge of the two plates. One advantage of the Astrolabe is that you do not need a clear horizon to use the instrument, you do need a clear horizon when measuring the height of the sun or Polaris with other navigational instruments. It is not a accurate tool at sea because of the difficlty in keeping it steady in a rolling ship and high winds. The Portuguese explorers would take their Astrolabe ashore and set it up to avoid this problem, this is what they did when they were mapping the coast of Africa in their early exploration. Using it at sea could result in errors.
Using it at sea could result in errors as much as five degrees, or 300 miles. However, ashore, it would be much more accurate, certainly less than one-half degree, or 30 miles. The seagoing version might be 6" in diameter, where as the one they took ashore (and which would be awkward to use at sea) might be two feet in diameter, making it more accurate and easier to read.
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