The celestial sphere is the sphere of viewing directions around an observer; distances to objects are suppressed. Great circles define coordinate reference planes.
An observer’s celestial meridian is the great circle containing the zenith and the celestial poles. Its upper half passes through the local north and south directions.
The celestial poles are the two intersections of the rotation axis of Earth with the celestial sphere. The northern pole has declination .
The celestial equator is the great circle perpendicular to the rotation axis of Earth; declination measures angular displacement from it.
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