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# What is Precession?

Article Details
• Written By: Jason C. Chavis
• Edited By: Bronwyn Harris
2003-2018
Conjecture Corporation
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Any alteration in the position of a celestial body's rotation axis is known as precession. Astronomical bodies naturally feature slow changes in both their rotational rate and orbital status around other gravitationally-strong bodies. Two types of precession exist within the realm of astronomy: torque-free and torque-induced. According to the mathematical principles of torque-free precession, the rotating axis is different from the ellipse of an object. In the case of torque-induced precession, the object maintains an alternating rotation.

The most common example of this phenomenon can be seen in the rotation of the Earth itself. Known primarily as precession of the equinoxes, the axial spinning of the Earth essentially traces out the shape of a cone as it orbits the Sun. This occurs within the Earth's orbit every 25,800 years and results in the slow change of the location of the stars in the night's sky. While the stars themselves do not actually change position, their location as compared to coordinates on the planet's surface alter. Observers of this shift on the Earth's crust can identify only a one degree change every 72 years.

In addition to the actual changes in rotation, the changes of inclination can affect precession. Inclination is basically the angle in which an astronomical body relates to a certain other body. In the case of Earth, the planet's inclination in the solar system is essentially aligned with Jupiter. However, this inclination will change due to natural drifting over the course of roughly 100,000 years.

The reason behind the changes in both rotation and inclination are due to the oval-shaped orbit of planets and other bodies. Gravity pulls objects towards larger objects in an imperfect way, resulting in the top-like spin of bodies. As the body spins, it causes resistive forces against the gravitational pull, resulting in a oblong orbit. The combination of these phenomena are believed to be the causes of such incidents on Earth as the Ice Ages, according to the field of precession astronomy.

The first understanding of precession was identified in 150 BC by Hipparchus, a Greek astronomer. However, studies have shown that other ancient cultures, such as the Mayans, were aware of the phenomenon as well. Modern theories involving the rotation and inclination of the Earth were established by the physical laws of Isaac Newton, who defined the effects of gravity from both the Sun and the Moon.