## Glossary term: Hour Angle

Description: The hour angle is the angle between an object's hour circle and the observer's meridian.

As seen from Earth, all the various positions in the sky together form what appears to be a distant sphere with Earth at its center. The points in the sky directly above Earth's equator form the celestial equator on that sphere. The point directly above Earth's geographic North Pole is the celestial North Pole, and that above Earth's South Pole, the celestial South Pole.

Just like geographers define geographic longitude and latitude on Earth's surface, one can define longitude and latitude on the celestial sphere. The meridian of an observer corresponds to the observer's geographical meridian (the circle whose center is the Earth's center, and which intersects the North Pole, South Pole, and the observer's position), projected onto the celestial sphere. It intersects the north point on the observer's horizon, the zenith, and the south point. The projected meridian that passes through a given celestial object is called that object's hour circle. The hour angle is the angle between the object's hour circle and the observer's meridian. As time passes, the hour angle changes: An hour angle of zero corresponds to the star's highest position (its upper culmination) in the sky. As the star moves towards the western horizon, the hour angle increases. As the hour angle approaches 360 degrees, the star approaches its next upper culmination. Note the time between upper culminations is one sidereal day, this is roughly four minutes shorter than a solar day. Because of this direct connection with time, the hour angle is usually stated in hours, not in degrees, with 360 degrees corresponding to 24 hours. The hour angle can be used to compute the time until an object's upper culmination. This is useful to astronomers planning their observations: at or near upper culmination, when it is farthest from the horizon, is a particularly good time to observe an object.

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