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| '''Right ascension''' ('''RA'''; symbol α: '''Greek letter ]'''; '''celestial longitude''') is the ] term for one of the two ]s of a point on the ] when using the ]. The other coordinate is the ]. | '''Right ascension''' ('''RA'''; symbol α: '''Greek letter ]'''; '''celestial longitude''') is the ] term for one of the two ]s of a point on the ] when using the ]. The other coordinate is the ]. | ||
| == Explanation == | == Explanation == | ||
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| RA is comparable to ], measured from a zero point known as the ''']'''. RA is measured in hours, minutes, and seconds. Being closely tied with ], it is both a unit of time and of angle. An '''hour of right ascension''' is equal to 15 ], a '''minute of right ascension''' equal to 15 ], and a '''second of right ascension''' equal to 15 ]. An alternative measure, used in navigation, is Sidereal Hour Angle. The main difference being that RA is measured Eastward, and SHA is measured Westward. | RA is comparable to ], measured from a zero point known as the ''']'''. RA is measured in hours, minutes, and seconds. Being closely tied with ], it is both a unit of time and of angle. An '''hour of right ascension''' is equal to 15 ], a '''minute of right ascension''' equal to 15 ], and a '''second of right ascension''' equal to 15 ]. An alternative measure, used in navigation, is Sidereal Hour Angle. The main difference being that RA is measured Eastward, and SHA is measured Westward. | ||
| RA can be used to determine a ]'s location and to determine how long it will take for a star to reach a certain point in the ]. For example, if a star with RA = 01:30:00 is at your ], then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later. |
RA can be used to determine a ]'s location and to determine how long it will take for a star to reach a certain point in the ]. For example, if a star with RA = 01:30:00 is at your ], then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later. | ||
| == Celestial longitude for other planets == | |||
| {{mainarticle|planetocentric longitude]] | |||
| ==See also== | ==See also== | ||
Revision as of 04:43, 16 December 2005
Right ascension (RA; symbol α: Greek letter alpha; celestial longitude) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.
Explanation
RA is comparable to longitude, measured from a zero point known as the vernal equinox point. RA is measured in hours, minutes, and seconds. Being closely tied with sidereal time, it is both a unit of time and of angle. An hour of right ascension is equal to 15 degrees of arc, a minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc. An alternative measure, used in navigation, is Sidereal Hour Angle. The main difference being that RA is measured Eastward, and SHA is measured Westward.
RA can be used to determine a star's location and to determine how long it will take for a star to reach a certain point in the sky. For example, if a star with RA = 01:30:00 is at your meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later.
Celestial longitude for other planets
{{mainarticle|planetocentric longitude]]
See also
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