This classic method of navigation, used most commonly in the open sea, the navigator uses celestial bodies that have been identified and grouped into constellations since ancient times. Celestial navigation makes possible voyages across thousands of miles of unmarked water, but its one great limitation is that poor visibility, caused by clouds, fog, rain, snow, mist, or haze, may prevent the essential sightings of celestial bodies.
A coordinate system of positions similar to the earth's coordinates of latitude and longitude has been adopted to describe the position of heavenly bodies. This system consists of Declination, which corresponds to terrestrial latitude, and Hour Angle, which corresponds to terrestrial longitude. For practical purposes of navigation, the position of stars relative to one another is regarded as fixed in the classical sphere; the motion of the sun, the moon, and the planets is indicated in this system as a mean rate of progression across the sphere.
The principal maritime nations publish yearly Nautical Almanacs that tabulate the coordinates of celestial bodies used in navigation at any particular time. The tables also provide other pertinent astronomical information.
To use the nautical almanac, the navigator must establish the time of an observation accurately by means of the chronometer. The Measurement of Time is based on the rotation of the earth and the consequent imaginary rotation of celestial bodies around the earth. In navigation, the primary system of time is based on the apparent movement of the sun westward at 15° of longitude per hr. Thus, a time difference is established between two places on the surface of the earth based on their difference of longitude. The longitude of New York City, for example, is roughly 75° West and that of Greenwich, England, is 0°. New York is therefore 5 hours to the west of Greenwich.
The navigational triangle, or Astronomical Triangle, which constitutes the most important part of celestial navigation, is a spherical triangle, the three points of which represent the position of the observer, the geographical position of the celestial body, and the earth's pole that is nearest to the observer. The solution of such a triangle provides the basis for the derivation of an astronomical line of position. Spherical Trigonometry was formerly required to solve such a problem, but this triangle can today be solved simply by using the nautical almanac in conjunction with one of several short tabular methods. The tabular methods include precomputed solutions of the astronomical triangle to accommodate any position of the observer and any celestial body observed.
In the most modern approach to celestial navigation, the Circle of Equal Altitude and the astronomical position line are used in conjunction with the solution of the navigational triangle. The circle of equal altitude is a circle on the surface of the earth, and at every point on this circle the altitude of a given celestial body is the same at a given instant.