Hour circle

Part of celestial coordinate system

In astronomy, the hour circle is the great circle through a given object and the two celestial poles. Together with declination and distance, it determines the location of any celestial object. As such, it is a higher concept than the meridian as defined in astronomy, which takes account of the terrain and depth to the centre of Earth at a ground observer's location. The hour circles, specifically, are perfect circles perpendicular to the celestial equator. By contrast, the declination of an object viewed on the celestial sphere is the angle of that object to/from the celestial equator.

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