Greenwich Hour Angle (GHA)

Greenwich Hour Angle is the angular distance of a body’s meridian measured westward from the Greenwich meridian, 0 to 360 degrees. It is the primary positional value the Nautical Almanac tabulates hourly for the Sun, Moon, planets, and Aries, with increments-and-corrections pages for the minutes and seconds. For a star, GHA equals GHA Aries plus the star’s sidereal hour angle. Subtracting the observer’s west longitude (or adding east longitude) converts GHA to the local hour angle used in sight reduction. With declination, GHA fixes the body’s geographical position, the point on Earth directly beneath it.