The determination of a fix of a vessel, anywhere on the earth’s surface, including those regions about the poles, is founded on a very simple elementary geometrical principle that can be readily understood by those who are not navigators. The entire process consists merely of taking with a marine sextant two different observations of either the sun, moon, planets, or stars, to measure accurately the true altitude of a celestial body above the horizon, at some particular given instant of Greenwich civil time. What that observation really gives is, in effect, the zenith distance of the heavenly body. The data in the Nautical Almanac for this particular observation are the celestial body’s co-ordinates of declination, or the body’s angular distance north or south of the equinoctial or equator, and its other co-ordinate, the Greenwich hour angle, which is the arc of longitude measured westward from the prime meridian of Greenwich. This enables a point on the earth’s surface which has the heavenly body in its zenith to be readily plotted on a map or chart.
Polar Navigation with H.O. Publication 214
By E. B. Collins