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Stars, planets, moons and other heavenly bodies seem to move through the sky and observers have developed conventions for describing the position of these bodies at a given time. A body in the sky appears to be located on a so-called celestial sphere centered on the observer. The position of a body may be described by two coordinates, the longitude and latitude of the position on the celestial sphere. There are at least three basic frames of reference for specifying the poles, the equator, and the prime meridian of the celestial sphere.

  1. The equatorial or geocentric system places the equator of the celestial sphere in the plane of the earth's equator, thereby making the poles coincide with the geographic poles of the earth's rotation.
  2. The ecliptic or heliocentric system places the equator of the celestial sphere in the plane of the earth's rotation about the sun.
  3. The galactic system locates the equator of the celestial sphere in the plane of the milky way. The milky way is a relatively flat galaxy and can be represented by a plane.

The planes through the geocentric equator and the heliocentric equator trace out great circles on the celestial sphere and these great circles intersect at the point that is called the First Point of Aries or FPA. This point, FPA, is the (0,0) point for both the equatorial and ecliptic systems. The equatorial and ecliptic poles are separated by an angle of 23 degrees, 27 minutes, 8.26 seconds or 0.409318496 radians that is called the obliquity of the ecliptic. The galactic pole is separated from the equatorial pole by an angle of 62.6 degrees or 1.0926 radians that is called the obliquity of the galactic plane. The obliquity of the ecliptic is usually represented by lower case epsilon and the galactic obliquity by upper case E. The (0,0) point of the galactic system is fixed at (192.25 deg, 0) in the equatorial system.

In conjunction with an experiment on Zodiacal Light, NASA Memorandum 53943 was prepared that carefully defined the transformation between coordinates in each of these systems. The source code for this program was taken from this document; it was not released by NASA through COSMIC.

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