The equator is defined as a great circle on the earth's surface formed by passing a plane perpendicular to the earth's axis midway between the poles. A meridian is any great circle on the earth's surface formed by passing a plane containing the earth's axis through the earth perpendicular to the equator; the prime meridian is the upper branch of the meridian which passes through the site of the Royal Observatory at Greenwich, England. The latitude of a position is defined as the angular distance measured from the equator northward or southward through 90 degree's the direction of measurement is indicated by placing a N (north) or S (south) after the angular measure. The longitude of a position is the angular distance measured from the prime meridian eastward or westward through 180 degree's the direction of measurement is indicated by placing a E (east) or W (west) after the angular measure. Each degree of latitude or longitude is subdivided into 60 minutes, and each minute is subdivided into either 60 seconds or tenths of a minute.
Every degree of arc of a great circle drawn on the earth's surface is assumed for navigational purposes to 60 nautical miles of distance, and every minute of a great circle arc one nautical mile.
Any position on the earth can be located by giving its terrestrial coordinates, any heavenly body on the celestial sphere can be located by giving its celestial coordinates. An hour circle is a great circle on the celestial sphere perpendicular to the celestial equator and passing through both celestial poles.
The 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°.
Declination is labeled with the prefix N (north) or S (south) to indicate the direction of measurement.
The celestial equivalent of longitude is the hour angle. The hour angle is an angular distance measured laterally along the celestial equator in a westerly direction through 360°.
If it is desired to locate a body on the celestial sphere relative to the location of Aries, the hour circle of Aries is used as a reference for the measurement of the hour angle. Hour angles are measured in a westerly direction from the hour circle of Aries to the hour circle of the body,this is called sidereal hour angles, abbreviated SHA.
In celestial navigation, you have to locate a body on the celestial sphere relative to Aries, but also have to locate a body relative to a given position on earth at a given time.
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