<P> In astronomy, axial precession is a gravity - induced, slow, and continuous change in the orientation of an astronomical body's rotational axis . In particular, it can refer to the gradual shift in the orientation of Earth's axis of rotation, which, similar to a wobbling top, traces out a pair of cones joined at their apices in a cycle of approximately 26,000 years . The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis--nutation and polar motion--are much smaller in magnitude . </P> <P> Earth's precession was historically called the precession of the equinoxes, because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the yearly motion of the Sun along the ecliptic . This term is still used in non-technical discussions, that is, when detailed mathematics are absent . Historically, the discovery of the precession of the equinoxes is usually attributed in the west to the Hellenistic - era (second - century BCE) astronomer, Hipparchus, although there are claims of its earlier discovery, such as in the Indian text, Vedanga Jyotisha, dating from 700 BC . With improvements in the ability to calculate the gravitational force between and among planets during the first half of the nineteenth century, it was recognized that the ecliptic itself moved slightly, which was named planetary precession, as early as 1863, while the dominant component was named lunisolar precession . Their combination was named general precession, instead of precession of the equinoxes . </P>

Because of the precession of the earth's axis