# Celestial Navigation

Celestial Navigation

This Demonstration illustrates the basic principle behind the practice of celestial navigation.

Using a sextant, the navigator measures the altitudes of two celestial bodies. The altitude is the angle of a body above the horizon. The bodies may be the Sun, Moon, planets, or stars.

The navigator knows the current date and time at the Greenwich meridian. Looking up data published in nautical or air almanacs, the navigator finds the geographical coordinates of the substellar points of the two bodies. As they were at the time of altitude measurement.

The geographical coordinates for each body has two elements. The longitudinal coordinate. And the latitudinal.

In this Demonstration, the longitudinal coordinate is denoted GHA, which is the abbreviation used in almanacs for Greenwich hour angle.

The latitudinal coordinate is denoted declination, also as used in the almanacs.

Given altitude, GHA, and declination for the bodies, it is possible to calculate the navigator's position at (or near) the Earth's surface. In general, two positions satisfy the observed and published data. The navigator can decide which position to choose, taking into account the past history of the voyage.