Precession of the Earth's axis | Wikipedia audio article
Channel: wikipedia tts
Date: 2019-06-16

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Axial_precession


00:03:24 1 Nomenclature
00:05:27 2 Effects
00:09:19 3 History
00:09:28 3.1 Hellenistic world
00:09:37 3.1.1 Hipparchus
00:11:23 3.1.2 Ptolemy
00:13:03 3.1.3 Other authors
00:14:10 3.2 Alternative discovery theories
00:14:20 3.2.1 Babylonians
00:14:56 3.2.2 Maya
00:15:30 3.2.3 Ancient Egyptians
00:18:34 3.3 Indian views
00:22:09 3.4 Chinese astronomy
00:22:35 3.5 Middle Ages and Renaissance
00:24:49 3.6 Modern period
00:25:28 4 Hipparchus's discovery
00:33:51 5 Mithraic question
00:39:14 6 Changing pole stars
00:42:19 7 Polar shift and equinoxes shift
00:45:54 8 Cause
00:49:53 9 Equations
00:56:21 10 Values
01:04:03 11 See also
01:04:42 12 Notes
01:10:47 13 Bibliography
01:13:32 14 External links
01:14:03 Notes
01:14:12 Bibliography
01:20:20 External links



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SUMMARY
=======
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 in a cycle of approximately 25,772 years. This is similar to the precession of a spinning-top, with the axis tracing out a pair of cones joined at their apices. 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.
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 BC) 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 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.
Lunisolar precession is caused by the gravitational forces of the Moon and Sun on Earth's equatorial bulge, causing Earth's axis to move with respect to inertial space. Planetary precession (an advance) is due to the small angle between the gravitational force of the other planets on Earth and its orbital plane (the ecliptic), causing the plane of the ecliptic to shift slightly relative to inertial space. Lunisolar precession is about 500 times greater than planetary precession. In addition to the Moon and Sun, the other planets also cause a small movement of Earth's axis in inertial space, making the contrast in the terms lunisolar versus planetary misleading, so in 2006 the International Astronomical Union recommended that the dominant component be renamed the precession of the equator, and the minor component be renamed precession of the ecliptic, but their combination is still named general precession. Many references to the old terms exist in publications predating the change.