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In astronomy, precession refers to the movement of the rotational axis of a body, such as a planet, with respect to inertial space. In particular, it refers to the precession of the Earth's rotational axis, also called the precession of the equinoxes. (The term "precession" may also refer to the rotational movement through space of the apsides of a body's orbit; this usage is explained later in the article.) Like a wobbling top, the orientation of the Earth's axis is slowly but continuously changing, tracing out a conical shape in a cycle of approximately 25,765 years[citation needed] (the so called Great or Platonic year, and also the determining factor in the length of an astrological age). This movement is caused by the gravitational forces of the Sun and the Moon, and to a lesser extent other bodies, on the equatorial bulge of the spinning Earth. The term "precession" typically refers only to this largest periodic motion; other changes in the alignment of Earth's axis — nutation and polar motion — are very much smaller in magnitude. In ancient times the precession of the equinox referred to the motion of the equinox relative to the background stars in the zodiac; this is equivalent to the modern understanding. It acted as a method of keeping time in the Great year.[1] Historically, Hipparchus is credited with discovering that the positions of the equinoxes move westward along the ecliptic compared to the fixed stars on the celestial sphere. The exact dates of his life are not known, but astronomical observations attributed to him date from 147 BC to 127 BC and were described in his writings, none of which are known to survive.
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