Hour circle


In astronomy, the hour circle, which together with declination and distance determines the location of any celestial object, is the great circle through the object and the two celestial poles. 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.
The location of stars, planets, and other similarly distant objects is usually expressed in the following parameters, one for each of the three spatial dimensions: their declination, right ascension, and distance. These are as located at the vernal equinox for the epoch stated.
A meridian on the celestial sphere matches an hour circle at any time. The hour circle is a subtype whereby it is expressed in hours as opposed to degrees, radians, or other units of angle. The hour circles make for easy prediction of the angle between the observation of two objects at the same, or similar declination. The hour circles are measured in hours ; one rotation is equivalent to 24 hours; 1 hour is equivalent to 15°.
An astronomical meridian follows the same concept and, almost precisely, the orientation of a meridian on a globe.