Stars Ī star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change gradually due to precession of the equinoxes and proper motion, and cyclically due to annual parallax. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, and B1950.0. The prefix "J" indicates that it is a Julian epoch. The currently used standard epoch is J2000.0, which is Januat 12:00 TT. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. Therefore, equatorial coordinates (including declination) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. The Earth's axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years.
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