Hour circle

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Diagram illustrating the definition of the hour circle of a star.

In astronomy, the hour circle of a celestial object is the great circle through the object and the celestial poles. It is perpendicular to the celestial equator. The declination of an object as observed on the celestial sphere is the angle measured along the hour circle of that object to the celestial equator.[1] In other words, it is analogous to meridians or longitudes on a globe. A meridian on the celestial sphere matches an hour circle at any time. So called because the angle between the planes of the hour circles (or the difference in longitudes) of two objects is sometimes measured in hours (or hours, minutes, and seconds), one revolution (360°) being equivalent to 24 hours, or 1 hour being equivalent to 15°.

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  1. ^ Wakker, Prof.ir. K.F. (March 2010). AE4874-I Astrodynamics part I. Delft University of Technology.