Right Ascension

Dec_and_RightAscensionThe vertical, or north-south, lines on the celestial sphere are known as right ascension, and are marked not in degrees, minutes and seconds, but in hours, minutes and seconds. Lines of right ascension are somewhat related to longitude, but since the Earth turns about its axis within the celestial sphere, the relationship is not fixed. The difference in the units of measure, degrees of longitude vs hours of right ascension, accounts for the changing relationship between the “fixed” surface of the Earth  and the  celestial sphere that “rotates” across the sky.

There are twenty-four hours of right ascension on the celestial sphere, which is related to the length of a day on the surface of the Earth. Two different hour lines of right ascension are one hour, or 15° apart. Fifteen degrees is exactly how much the celestial sphere appears to rotate in one hour. Twenty-four right ascension hours spaced 15° apart represent the 360° circle around the celestial sphere.

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