The Six Conditions of the Equatorial Coordinate System

1. The physical basis for the Equatorial System is, also, the Earth's rotation.
2. The Fundamental Great Circle (FGC) of the system is the Celestial Equator (the central projection of the Terrestrial Equator upon the interior of the Celestial Sphere).
3. The "poles" of the system are the North Celestial Pole (NCP, denoted " P " in the accompanying diagram, to the right) and the South Celestial Pole (SCP, denoted " P' " in the figure).
4. The secondary great circles are "hour circles."
5. The origins associated with each coordinate are:
   • the latitudinal coordinate, similar to Terrestrial Latitude, is zeroed at the Celestial Equator which is the central projection of the Terrestrial Equator upon the interior of the Celestial Sphere), increasing positively towards the NCP, and decreasing negatively towards the SCP.
   • there are two longitudinal coordinates in this system:
     1. the first coordinate, analagous to Terrestrial Longitude, increases clockwise along the FGC  from the Equator Point (Σ) to the point 'M.'
     2. the second coordinate increases counter-clockwise along the FGC from the Vernal Equinox to the point 'M.'
6. The systemic coordinates are:
   • the latitudinal coordinate is called the "Declination" (δ), expressed in arc-units: - 90° ≤ δ ≤ + 90°;
   • the longitudinal coordinates are called ...
     1. the "Local Hour Angle" (LHA), and is usually expressed in units of time: 0^h≤  LHA ≤ 24^h;
     2. the "Right Ascension" (α), and is usually expressed in units of time: 0^h≤ α ≤ 24^h.