hour angle

hour angle in astronomy, a coordinate in the equatorial coordinate system . The hour angle of a celestial body is the angular distance, expressed in hours, minutes, and seconds (one hour equals 15 degrees), measured westward along the celestial equator from the observer's celestial meridian to the hour circle of the object being located. The hour angle is used in measuring astronomical time; local sidereal time is equal to the hour angle of the vernal equinox.

hour angle (HA) The angle between the observer's meridian and the hour circle of a celestial object, measured clockwise along the celestial equator. The observer's meridian (the great circle joining the celestial pole to the zenith) is therefore the meridian of zero hour angle. The hour angle of any object increases as the Earth rotates, and it can be expressed as a time by equating 360° with 24 hours. For example, an hour after an object crosses the meridian its hour angle is 1 h, and so on. It is sometimes used as a coordinate in place of right ascension. The sum of the hour angle and right ascension is equal to the observer's sidereal time. See also greenwich hour angle.

hour angle, the angular distance west of a celestial meridian. It is normally specified as Local (LHA), Greenwich (GHA), or Sidereal (SHA), that is, measured westward from the observer's celestial meridian, or that of Greenwich, or that of the hour circle of the spring equinoctial. In sight reduction, LHA = GHA - W.Long. or GHA + E.Long.

Mike Richey