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Right ascension (RA) is an astronomical term associated with the equatorial coordinate system. 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 zenith, then a star with RA = 20:00:00 will be directly overhead in 18.5 sidereal hours.
See also: declination