Editing Right ascension (section)

==History==
{{Refimprove section|date=May 2012}}

[[File:Old RA diagram.png|thumb|400px|How '''right ascension''' got its name. Ancient astronomy was very concerned with the rise and set of celestial objects. The ''ascension'' was the point on the [[celestial equator]] (red) which rose or set at the same time as an object (green) on the [[celestial sphere]]. As seen from the equator, both were on a [[great circle]] from pole to pole (left, ''sphaera recta'' or right sphere). From almost anywhere else, they were not (center, ''sphaera obliqua'' or oblique sphere). At the poles, objects did not rise or set (right, ''sphaera parallela'' or parallel sphere). An object's right ascension was its ascension on a right sphere.<ref>Blaeu (1668), p. 40–41.</ref>]]

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 catalog]]s in [[Ecliptic coordinate system|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).
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[[File:Stars and ra.png|thumb|500px|center|The entire sky, divided into two halves. '''Right ascension''' (blue) begins at the [[Equinox|March equinox]] (at right, at the intersection of the [[ecliptic]] (red) and the [[celestial equator|equator]] (green)) and increases eastward (towards the left). The lines of right ascension (blue) from pole to pole divide the sky into 24 hours, each equivalent to 15°.]]