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Right ascension (abbreviated RA; symbol α) is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.
An old term, right ascension (Latin, ascensio recta) refers to the ascension, or the point on the celestial equator which rises with any celestial object, as seen from the Earth's equator, where the celestial equator intersects the horizon at a right angle. It is contrasted with oblique ascension, the point on the celestial equator which rises with a celestial object as seen from almost anywhere else on Earth, where the celestial equator intersects the horizon at an oblique angle.
Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an equator. Right ascension is measured from the vernal equinox or the first point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is located in the constellation Pisces. Right ascension is measured continuously in a full circle from that equinox towards the east.
Any units of angular measure could have been chosen for right ascension, but it is customarily measured in hours ( h ), minutes ( m ), and seconds ( s ), with 24h being equivalent to a full circle. Astronomers have chosen this unit to measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the Earth rotates. The highest point in the sky, called meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360 degrees of arc°, 1⁄24 of a circle is measured as 1h of right ascension or 15°; 1⁄(24×60) of a circle is measured as 1m of right ascension or 15 minutes of arc (also written as 15'); and 1⁄(24×60×60) of a circle contains 1s of right ascension or 15 seconds of arc (also written as 15"). A full circle, measured in right ascension units, contains 24×60×60 = 86,400s, or 24×60 = 1,440m, or 24h.
Because right ascensions are measured in hours (of rotation of the Earth), they can be used to time the positions of objects in the sky. For example, if a star with RA = 01h 30m 00s is on the meridian, then a star with RA = 20h 00m 00s will be on the meridian 18.5 sidereal hours later.
Sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Usually measured in degrees ( ° ), it is the complement of right ascension with respect to 24h. It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian.
|Unit||Value||Symbol||Sexagesimal system||In radians|
|Hour||1⁄24 circle||( h )||15°||π⁄12 rad|
|Minute||1⁄60 hour, 1⁄1,440 circle||( m )||1⁄4°, 15'||π⁄720 rad|
|Second||1⁄60 minute, 1⁄3,600 hour, 1⁄86,400 circle||( s )||1⁄240°, 1⁄4', 15"||π⁄43200 rad|
The Earth's axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates (including right ascension) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.
The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, and B1950.0.
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The concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made their star catalogs in ecliptic coordinates, and the use of RA was limited to special cases.
With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do that is to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis. A motorized clock drive often is used with an equatorial mount to cancel out the Earth's rotation. As the equatorial mount became widely adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles. The first star catalog to use right ascension and declination was John Flamsteed's Historia Coelestis Britannica (1712, 1725).