Right ascension

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The right ascension (RA or α) of an object in the sky is the angular distance clockwise from the point where the sun crosses the celestial equator at the moment of the vernal equinox to the azimuth of the object. The right ascension is used with the declination in the equatorial coordinate system.

Right ascension is measured in units of time, with one day of 24 hours equal to a full circle. Right ascension is always reported as a positive number of hours, minutes, and seconds, with finer measurements using decimal fractions of seconds. As there are 360 degrees of arc in the full circle, and 24 hours of right ascension, one hour of right ascension is equal to 15 degrees of arc, one minute of RA is equal to 0.25 degrees or 15 minutes of arc, and one second of RA is equal to 0.0041666 degrees or 15 seconds of arc.

The advantage of using time, rather than conventional angular measurements, is that an observer pointed in a fixed direction will move his viewpoint by one hour of right ascension in one hour.