In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particular, Earth's axial precession is the gradual shift in the orientation of Earth's axis of rotation. It has a cycle of approximately 26,000 years, tracing out a double cone with a half-aperture of about 23.4°, an angle known as the obliquity of the ecliptic.Hohenkerk, C.Y., Yallop, B.D., Smith, C.A., & Sinclair, A.T. "Celestial Reference Systems" in Seidelmann, P.K. (ed.) Explanatory Supplement to the Astronomical Almanac. Sausalito: University Science Books. p. 99. This is similar to the precession of a spinning top. 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. Historically,Astro 101 – Precession of the Equinox , Western Washington University Planetarium. Retrieved 30 December 2008 the discovery of the precession of the equinoxes is usually attributed in the West to the 2nd-century-BC astronomer Hipparchus. 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.Robert Main, Practical and Spherical Astronomy (Cambridge: 1863) pp.203–4. 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, net northward or southward pull of major nearby planets on Earth and its orbital plane (the ecliptic), causing that plane to shift slightly relative to inertial space. The slight difference in planes of planets explains why despite the ecliptic plane being so-called, eclipses of planets in the form of exact conjunctions are very rare. 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 between the terms lunisolar and 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.
Nomenclature
The term "precession" as it is used in astronomy generally describes the observed but exceedingly slow 26,000 year changing location of the place where the Sun appears against the background stars when it travels northward across the equator, at the moment of the March equinox. The stars oppositely seem to be moving retrograde across the sky, relative to the false-perspective that the stars move, rather than the equinox locations moving. In physics the term "precession" is generally used slightly differently to instead describe the analogous mechanical process of spinning tops shifting their orientation to their surroundings when tipped slightly while spinning.
The term "precession" is derived from the Latin praecedere ("to precede, to come before, to be earlier"). As viewed from Earth, the stars are seen to drift slightly faster than one full revolution across the sky from east to west, at about 15 degrees per hour, due to the Earth's actual diurnal motion in the opposite direction. The stars also rotate around the Earth's axis even more slowly, once every year (at about 1 degree per day), which is a mirror reflection of the Earth's orbital motion around the Sun. At the same time the stars can be observed to move yet again more slowly in the same direction as their annual circuit of the Earth's polar axis, very slightly faster than their normal yearly motion. That later rate is approximately 50 arcseconds per year – 1 degree every 72 years or one full rotation about every 26,000 years – a phenomenon known as the "precession of the equinoxes".
In describing the motion they see, astronomers generally have shortened the term to simply "precession". Physicists also use the term "precession" to describe the cause of the motion, which has led to some confusion between the phenomenon observed in the sky and the imbalance of torques that causes tops to wobble. That distinction matters for astronomy, because some precessional motions are actual and real, whereas others are only apparent motion caused by the false perception that the background stars drift and the Earth's equinoxes are fixed, a consequence of viewing the sky from the perspective of a very slowly moving reference frame (the Earth's axis of rotation and its orbital axis). The issue is further obfuscated by the fact that modern astronomers are almost entirely physicists, and mix the astronomical and physical vocabularies, whose fine distinctions that might be more easily understood if kept separate.
Effects
The precession of the Earth's axis has two main observable effects.
The positions of the south and north celestial poles move in quite large circles against the space-fixed backdrop of stars, completing one circuit in approximately 26,000 years. Thus, while today the star Polaris in Ursa minor lies approximately at the north celestial pole, this will change over time, and other stars will become the "north star". In approximately 3,200 years, the star Gamma Cephei in Cepheus will next succeed the other star which has great apparent magnitude. The south celestial pole currently lacks a bright star to mark its position, but over time precession also will cause bright stars to become South Stars. As the celestial poles shift, there is a corresponding gradual shift in the apparent orientation of the whole star field, as viewed from a particular position on Earth. The position of the Earth in its orbit at the solstices, equinoxes, and any other annual time defined by exact point in a season (or if preferred such it may be within the Tropics, the length of day and night), slowly changes at a near-constant rate. For example, suppose that the Earth's orbital position is marked at the June solstice, when the Earth's axial tilt is pointing at its most to the Sun in the northern hemisphere. At the exact time of one orbit later when the Sun returns to the same apparent position relative to the background stars, the chosen reference point in terms of axial tilt has happened 20 minutes beforehand: because of precession. Thus, the tropical year, measuring the cycle of seasons (for example, the time from solstice to solstice, or equinox to equinox), is about 20 minutes shorter than the sidereal year, which is simply measured by the Sun's apparent position against the other stars. After about 26 000 years, the difference amounts to a full year, so the positions of the seasons relative to the orbit are "back where they started". (Other effects also slowly change the shape and orientation of the Earth's orbit, and these, in combination with precession, create various cycles of differing periods; see also Milankovitch cycles. The magnitude of the Earth's tilt, as opposed to merely its orientation, also changes slowly over time, but this effect is not attributed directly to precession.)
For identical reasons, the apparent position of the Sun relative to the backdrop of the stars at some seasonally fixed time (such as a date in a Calendar year) slowly regresses a full 360° through an almost unwavering path traced by the earth's orbit, passing through 13 constellations, at the rate of about 50.3 seconds of arc per year, meaning 1 degree every 71.6 years.
At present, the rate of precession corresponds to a period of 25,772 years, so a tropical year is shorter than a sidereal year by 1,224.5 seconds
The rate itself varies somewhat with time (see Values below), so one cannot say that in exactly 25,772 years the Earth's axis will be back to where it is now.
For further details, see Changing pole stars and Polar shift and equinoxes shift, below.
History
Hellenistic world Hipparchus The discovery of precession usually is attributed to Hipparchus (190–120 BC) of Rhodes or Nicaea, a Greek astronomer. According to Ptolemy's Almagest, Hipparchus measured the longitude of Spica and other bright stars. Comparing his measurements with data from his predecessors, Timocharis (320–260 BC) and Aristillus (~280 BC), he concluded that Spica had moved 2° relative to the autumnal equinox. He also compared the lengths of the tropical year (the time it takes the Sun to return to an equinox) and the sidereal year (the time it takes the Sun to return to a fixed star), and found a slight discrepancy. Hipparchus concluded that the equinoxes were moving ("precessing") through the zodiac, and that the rate of precession was not less than 1° in a century, in other words, completing a full cycle in no more than 36,000 years.
Virtually all of the writings of Hipparchus are lost, including his work on precession. They are mentioned by Ptolemy, who explains precession as the rotation of the celestial sphere around a motionless Earth. It is reasonable to presume that Hipparchus, like Ptolemy, thought of precession in geocentric terms as a motion of the heavens, rather than of the Earth.
Ptolemy The first astronomer known to have continued Hipparchus's work on precession is Ptolemy in the second century AD. Ptolemy measured the longitudes of Regulus, Spica, and other bright stars with a variation of Hipparchus's lunar method that did not require eclipses. Before sunset, he measured the longitudinal arc separating the Moon from the Sun. Then, after sunset, he measured the arc from the Moon to the star. He used Hipparchus's model to calculate the Sun's longitude, and made corrections for the Moon's motion and its parallax.Evans 1998, pp. 251–255 Ptolemy compared his own observations with those made by Hipparchus, Menelaus of Alexandria, Timocharis, and Agrippa. He found that between Hipparchus's time and his own (roughly 265 years) the stars had moved 2°40', or 1° in 100 years (36" per year; the rate accepted today is about 50" per year or 1° in 72 years). Ptolemy stated that he made his own observations at the beginning of the reign of the emperor Antoninus Pius and he said Hipparchus had made his in the 50th year of the third Callippic cycle. Ptolemy wrote:
Ptolemy also confirmed that precession affected all fixed stars, not just those near the ecliptic, with the same period of 36,000 years.
Other authors Most ancient authors did not mention precession and, perhaps, did not know of it. For instance, Proclus rejected precession, while Theon of Alexandria, a commentator on Ptolemy in the fourth century, accepted Ptolemy's explanation. Theon also reports an alternate theory:
"According to certain opinions ancient astrologers believe that from a certain epoch the solstitial signs have a motion of 8° in the order of the signs, after which they go back the same amount. ..." (Dreyer 1958, p. 204)
Instead of proceeding through the entire sequence of the zodiac, the equinoxes "trepidated" back and forth over an arc of 8°. The theory of trepidation is presented by Theon as an alternative to precession.
Alternative discovery theories Babylonians