# hour angle

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## hour angle,

in astronomy, a coordinate in the equatorial coordinate system**equatorial 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.

**.....**Click the link for more information. . 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 circle

**hour 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.

**.....**Click the link for more information. of the object being located. The hour angle is used in measuring astronomical time; local sidereal time

**sidereal 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.

**.....**Click the link for more information. 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

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

360° measured from the celestial meridian of the Greenwich to the hour circle of the body. 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 through360° 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.Want to thank TFD for its existence? Tell a friend about us, add a link to this page, or visit the webmaster's page for free fun content.

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