Hour circle
in astronomy, the great circle through a given object and the two celestial poles
In astronomy, the hour circle is the great circle through a given object and the two celestial poles. Together with declination and distance (from the planet's centre of mass), 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 (at right angles) 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 (thus ranging from +90° to −90°).
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 (epoch-fixed hour angle), and distance. These are as located at the vernal equinox for the epoch (e.g. J2000) 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.
“Hour circle” enters the record as in astronomy, the great circle through a given object and the two celestial poles. Crown Archives preserves that source wording while asking what Hour, circle and astronomy can confirm, complicate or overturn.
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This entry incorporates text from “Hour circle” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.