Principles of Celestial Navigation

Basics » Celestial Position

Similar to the geographic coordinate system of longitude and latitude, there is a coordinate system for the celestial sphere. In navigation, the celestial coordinate system uses Greenwich hour angle and declination to uniquely describe the positions of bright stars, the Sun, Moon, and planets.

schematic showing Earth sphere and celestial sphere

There is a celestial equator, which is an imaginary line on the sky directly above the Earth's equator.

Given a star in the sky, its Greenwich hour angle, often called GHA, is an angular measure along the celestial equator of how far West it is from the meridian at Greenwich, England. Its declination, often just called Dec, is an angular measure of how far North or South it is from the celestial equator. A star directly on the celestial equator has a declination of 0 degrees; one directly above the North pole is at declination 90 degrees North.

schematic showing Earth coordinates and celestial coordinates