Celestial navigation has been in use in one form or another since the beginning of time, and is the only sure way of finding the ship's position in the open sea without using the GPS even today.
Navigators find their position close to land using ‘fixes’. If one is due South of a lighthouse, for example, one is anywhere on a line drawn directly South from the lighthouse (a line of bearing). If there is another lighthouse in the vicinity, another line of bearing is drawn- where these two lines intersect is the ship’s position. For more complicated calculations over short distances, plane trignometry can be easily used to solve the problem of position fixing.
The problem is that there are no lighthouses in the open sea. Long ago, mariners discovered that they could instead use the stars and planets and obtain the ship’s position using spherical trignometry. They used a ‘spherical triangle’, an imaginary triangle on the surface of the earth with three points- the North Pole, the ‘Geographical Position’ of the celestial body (eg sun) and the ship’s approximate position (also called the Dead Reckoning Position).
The Geographical position of the body is the point at which a line drawn from the centre of the body and the centre of the earth touches the earths surface.
The sides of this spherical triangle thus become
1. A line drawn from the pole to our position. This is the same as 90 degrees minus the Latitude
2. A line drawn from the pole to the GP. This is the same as 90 degrees minus the body’s declination (angle in degrees above the celestial equator, similar to latitude and detailed in the Almanac.)
3. A line drawn from our position to the GP. Same as 90 degrees minus the calculated height of the body above the horizon, also called zenith distance.
This is the triangle which is solved.