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Right ascension (RA; symbol α : Greek letter alpha) 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. RA is comparable to longitude, measured from a zero point known as the vernal equinox point. RA is measured in hours, minutes, and seconds; with one hour equal to 15 degrees.
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.
See also: declination
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