Glossary term: Hour Angle

Description: The hour angle is one of the coordinates assigned to a sky position in the equatorial coordinate system, the second coordinate being the declination. The points in the sky that lie directly above Earth's equator form the celestial equator. For a star on the celestial equator, hour angle zero responds to the star's highest position above the horizon (its "culmination"); let us call that highest point on the equator the "equator culmination point" (not a standard term). To obtain the hour angle for any other star, project the sightline from us to that star to the equatorial plane. The angle between that projected line and our sightline to the equatorial culmination point is the hour angle. Any star on the equator rises at an hour angle of -90 deg. and sets at +90 deg.

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The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".