Celestial navigation is one of many forms of navigation involved with finding a position by means of plotting intersecting lines. This position is expressed in terms of latitude and longitude, coordinates on the earth's surface. Latitude (L) is the coordinate measured in a north or south direction using the equator as a starting point. The largest latitude possible is 90°, north or south, which would be at the poles.
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 are great circles passing through the earth's poles and the positions
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 is defined as having a specific latitude, north or south, and a specific longitude, east or west.
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 o 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 rather than the less accurate DR. All coordinates or angles will be expressed in degrees, minutes and tenths of minutes, remember 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, , 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.
