hour angle


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hour angle,

in astronomy, a coordinate in the equatorial coordinate systemequatorial coordinate system,
the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere. The celestial sphere is an imaginary sphere with the observer at its center.
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. The hour angle of a celestial body is the angular distance, expressed in hours, minutes, and seconds (one hour equals 15 degrees), measured westward along the celestial equator from the observer's celestial meridian to the hour circlehour circle,
in astronomy, a secondary axis in the equatorial coordinate system. The hour circle of a celestial body is the great circle on the celestial sphere that passes through both the body and the north celestial pole.
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 of the object being located. The hour angle is used in measuring astronomical time; local sidereal timesidereal time
(ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian.
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 is equal to the hour angle of the vernal equinox.

hour angle

(HA) Symbol: t. The angle measured westward along the celestial equator from an observer's meridian to the hour circle of a celestial body or point. It is usually expressed in hours, minutes, and seconds from 0h to 24h. It is thus measured in the same units but in the opposite direction to right ascension. The angle measured eastward along the equator from the meridian is sometimes called the meridian angle. Due to the daily apparent rotation of the celestial sphere, a celestial body's hour angle increases daily from 0h at the meridian; after six hours the hour angle is 6h and 24 hours later the celestial body again crosses the meridian. See also sidereal hour angle.

hour angle

[′au̇r ‚aŋ·gəl]
(astronomy)
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 vernal equinox, measured westward through 360°.

hour angle

hour angleclick for a larger image
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 vernal equinox, measured westward through 360°. At any instant, hour circles are coincident with particular celestial meridian. There are three types of hour angles: sidereal, Greenwich, and local. The sidereal hour angle is the angle of the celestial pole between the celestial meridian of the first point of the Aries and the celestial meridian of the body, measured westward from the celestial meridian of the first point of Aries. The Greenwich hour angle is the arc of the equatorial measured westward through
360° measured from the celestial meridian of the Greenwich to the hour circle of the body. The local hour angle is the angle at the celestial pole measured westward from the observer's meridian to the celestial meridian of the body.