The Celestial Coordinate System
Just as any position on the earth can be located by its terrestrial coordinates, any heavenly body on the celestial sphere can be located by its celestial coordinates. To form the celestial coordinate system, the terrestrial equator is projected outward onto the celestial sphere to form the celestial equator, sometimes called the equinoctial. The celestial equator is the reference for north-south angular measurements on the celestial sphere. In similar fashion, terrestrial meridians can be projected outward to the celestial sphere to form celestial meridians. Because of the apparent rotation of the celestial sphere with respect to the earth. A hour circle is a great circle on the celestial sphere perdicular to the celestial equator and passing through both celestial poles.
Every point on the celestial sphere has an hour circle passing through ,as the meridian passing through the observatory at Greenwich, was chosen as the reference for the lateral coordinate of a point on the terrestrial sphere, the hour circle passing through the point of Aries forms the reference for the lateral coordinate of a point on the celestial sphere. This hour circle is referred as the "hour circle of Aries."
Celestial equivalent of terrestrial latitude is declination, Declination is the angular distance of a point on the celestial sphere north or south of the celestial equator, measured through 90 degrees.
Declination is labeled with the prefix N (north) or S (south) to indicate direction of measurement; prefixes are used to differentiate declination from latitude.
celestial equivalent of longitude is the hour angle; it is defined the hour angle is an angular distance measured laterally along the celestial equator in a westerly direction through 360 .
If it desired to locate a body on the celestial sphere relative to Aries, the hour circle of Aries is used as a reference for measument of the hour angle. Hour angles, measured in westly direction from the hour circle of Aries to the hour circle body, are called sidereal hour angles, abbreviated SHA.
The purposes of celestial navigation, is to locate body on the celestial sphere relative to Aries, but also to locate a body relative to a given position on earth at a given time. To do this, two terrestrial meridians are projected onto the surface of the celestial sphere for use as references for hour angle measurements,the Greenwich meridian and the observer's meridian. The celestial meridians termed the Greenwich celestial meridian and the local celestial meridian. Hour angles measured relative to the Greenwich celestial meridian are called Greenwich hour angles, GHA, while those measured relative to the local celestial meridian are termed local hour angles, LHA. Both Greenwich hour angles and local hour angles are measured westward from a projected terrestrial meridian to a celestial hour circle moving westerly with the rotating celestial sphere. Both GHA and LHA values grow larger with time, increasing from 00 to 360° once each 24 hours; they relate the rotating celestial sphere to the meridians of the earth. Sidereal hour angles, on the other hand, are measured between two hour circles on the celestial sphere. When working on sights you will need the GHA of the STAR = GHA of ARIES + SHA of the STAR. Another coordinate is Right Ascension it is used when finding plants.
