Hour angle

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Hour angle

In astronomy, the hour angle is one of the coordinates used in the equatorial coordinate system for describing the position of a point on the celestial sphere.The hour angle of a point is the angle between the half plane determined by the Earth axis and the zenith (half of the meridian plane) and the half plane determined by the Earth axis and the given point. The angle is taken with minus sign if the point is eastward of the meridian plane and with the plus sign if the point is westward of the meridian plane.

The hour angle is usually expressed in time units, with 24 hours corresponding to 360 degrees.

The hour angle must be paired with the declination in order to fully specify the position of a point on the celestial sphere as seen by the observer at a given time.

Relation with the right ascension

The hour angle (HA) of an object is equal to the difference between the current local sidereal time (LST) and the right ascension ($alpha$) of that object:

:HAobject = LST - $alpha$object

Thus, the object's hour angle indicates how much sidereal time has passed since the object was on the local meridian. It is also the angular distance between the object and the meridian, measured in hours (1 hour = 15 degrees). For example, if an object has an hour angle of 2.5 hours, it transited across the local meridian 2.5 sidereal hours ago (i.e., hours measured using "sidereal time"), and is currently 37.5 degrees west of the meridian. Negative hour angles indicate the time until the next transit across the local meridian. Of course, an hour angle of zero means the object is currently on the local meridian.

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• Hour angle — Hour Hour, n. [OE. hour, our, hore, ure, OF. hore, ore, ure, F. heure, L. hora, fr. Gr. ?, orig., a definite space of time, fixed by natural laws; hence, a season, the time of the day, an hour. See {Year}, and cf. {Horologe}, {Horoscope}.] 1. The …   The Collaborative International Dictionary of English

• hour angle — n. Astron. the angle formed at the pole, or its arc on the celestial equator, between the hour circle of a celestial object and the observer s celestial meridian measured in hours, minutes, etc.: it represents the elapsed time since the object… …   English World dictionary

• hour-angle — hourˈ angle noun (astronomy) The angle (usu measured as time) between the declination circle of a body observed and the observer s meridian • • • Main Entry: ↑hour …   Useful english dictionary

• hour angle — The angular distance west of a celestial meridian or hour circle; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of a celestial meridian or hour circle and the hour circle of a celestial body or the …   Aviation dictionary

• hour angle — noun 1. the angular distance along the celestial equator from the observer s meridian to the hour circle of a given celestial body (Freq. 1) • Hypernyms: ↑distance 2. (astronomy) the angular distance of a celestial point measured westward along… …   Useful english dictionary

• hour angle — Astron. the angle, measured westward through 360°, between the celestial meridian of an observer and the hour circle of a celestial body. Cf. sidereal hour angle. [1830 40] * * * ▪ astronomy  in astronomy, the angle between an observer s meridian …   Universalium

• hour angle — valandų kampas statusas T sritis fizika atitikmenys: angl. hour angle vok. Stundenwinkel, m rus. часовой угол, m pranc. angle horaire, m …   Fizikos terminų žodynas

• hour angle — hour′ an gle n. astron. the angle, measured westward through 360°, between the celestial meridian of an observer and the hour circle of a celestial body • Etymology: 1830–40 …   From formal English to slang

• hour angle — noun Date: circa 1837 the angle between the celestial meridian of an observer and the hour circle of a celestial object measured westward from the meridian …   New Collegiate Dictionary

• Greenwich hour angle — hour angle measured from the meridian of Greenwich, England. * * * …   Universalium