Introduction to Celestial Navigation
Celestial navigation may be defined as the art of navigation with the aid of the sun, moon, planets, and the major stars. In order to practice this art the navigator has to be well versed in spherical trigonometry. Now with the inspection tables, which offer solutions of the spherical triangle, and programmable personal electronic calculators, you need little mathematical skill beyond the ability to do simple addition and subtraction. You should be familiar with concepts and assumptions which celestial navigation is based.
In celestial navigation, the earth is assumed to be a sphere, located at the center of the universe. The universe is assumed to be a second sphere of infinite radius concentric with the earth. It is called the celestial sphere, and all heavenly bodies are considered to be located on it.
The earth's rotation from west to east causes the celestial sphere to appear to rotate slowly in the opposite direction, causing the bodies to rise in the east, cross the observer's meridian, and then set in the west. Longitude (A) is measured along the equator in an east-west manner.
Since the equator is a great circle with no beginning or no end, a point along this circle must be established from which to measure.
Meridians, great circles passing through the earth's poles and the positions
in question, intersect the equator at right angles. On a Mercator projection chart or plotting sheet, meridians appear as vertical lines. Longitude is measured east OR west from the prime meridian with maximum measurement of 180°. Increasing measurement eastward would be moving to the right on a plotting sheet or chart, increasing measurement westward would be moving to the left.
A position on the earth can be defined as having a specific latitude, north or south, and a specific longitude, east or west. The celestial equivalent of latitude is declination (Dec.). It is the angular distance north or south of the celestial equator. It is expressed in degrees and minutes of arc, to the nearest tenth of a minute, and is labeled N or S to indicate the direction of measurement. The other celestial coordinate, equivalent to longitude on earth is hour angle. Greenwich hour angle (GHA) is the angular distance of a celestial body west of the celestial meridian of Greenwich. GHA is measured in arc from 0° to 360° and is stated in degrees and minutes to the nearest tenth.It is different than longitude which is measured east or west to 180 degrees.
A course is established to the nearest whole degree and is always written in three digits (316, 007, 068). Speed is written in units of nautical miles per hour or knots. One nautical mile (1.151 statute mile) is defined as being equal to one minute of latitude anywhere on earth. Distance, is measured on the latitude (vertical) scale of the plotting sheet. Time is referred to in the 24 hour clock manner and read as fifteen hundred (1500), eighteen twenty six (1826), twenty two thirty (2230) or zero seven nineteen (0719). For labelling purposes, time is written to the nearest minute.
DR refers to a position established by means of course and speed alone and is called the dead reckoning position. A fix is obtained by getting bearings or distances of terrestrial objects whose positions are known. When a fix is established, the new course and speed proceed from this more accurately known point than the less accurate DR. All coordinates or angles will be expressed in degrees, minutes and tenths of minutes, remembering that there are sixty minutes in a degree.
To establish a fix with piloting procedures, the navigator will choose a point at the intersection of two or more bearing lines (obtained by compass, radio direction finder, Loran, etc.), at the intersection of two or more circles of distance off (obtained by radar, or sextant altitude of the lighthouse, tower, etc. and use of Table 9 in American Practical Navigator, Volume II, by Bowditch), or at the intersection of a combination of these lines or circles.
