Precession

Precession. For a long period of time it has been observed that the positions of the celestial pole and the equator have altered. The pole of the equator, in fact, describes a small circle round the pole of the ecliptic, and the intersections of equator and ecliptic - the equinoxes - move backwards on the ecliptic through a distance rather less than one minute per year. Hence the sun has to travel about one minute less each year before he reaches the same equinox again, i.e. he reaches that point rather earlier every year. Hence the phenomenon is known as the "precession of the equinoxes." This precession has considerable effect upon the recorded positions of stars, since it increases their longitude by the amount of the precession, and effects a change in their declinations and right ascensions. It takes about 26,000 years for the equinox to make a complete revolution. The cause of this movement is the attraction of the sun and moon on the bulging part of the earth at the equator, which thus alters the direction of the axis. The phenomenon of precession was first noted by Hipparchus.