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Lunar precession
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Lunar precession is a term used for three different precession motions related to the Moon. First, it can refer to change in orientation of the lunar rotational axis with respect to a reference plane, following the normal rules of precession followed by spinning objects. In addition, the orbit of the Moon undergoes two further types of precessional motion: apsidal and nodal.
Axial precession
[edit]The rotational axis of the Moon also undergoes precession. Since the Moon's axial tilt is only 1.5° with respect to the ecliptic (the plane of Earth's orbit around the Sun), this effect is small. Once every 18.6 years,[1] the lunar north pole describes a small circle around a point in the constellation Draco, while correspondingly, the lunar south pole describes a small circle around a point in the constellation Dorado. Similar to Earth, the Moon's axial precession is westwards [2] - whereas Apsidal precession is in the same direction as the rotation (meaning apsidal precession is eastward).
Apsidal precession
[edit]
This kind of precession is that of the major axis of the Moon's elliptic orbit (the line of the apsides from perigee to apogee), which precesses eastward by 360° in approximately 8.85 years. This is the reason that an anomalistic month (the period the Moon moves from the perigee to the apogee and to the perigee again) is longer than the sidereal month (the period the Moon takes to complete one orbit with respect to the fixed stars). This apsidal precession completes one rotation in the same time as the number of sidereal months exceeds the number of anomalistic months by exactly one, after about 3,233 days (8.85 years).
Nodal precession
[edit]
Another type of lunar orbit precession is that of the plane of the Moon's orbit. The period of the lunar nodal precession is defined as the time it takes the ascending node to move through 360° relative to the vernal equinox (autumnal equinox in the Southern Hemisphere). It is about 18.6 years and the direction of motion is westward, i.e., in the direction opposite to the Earth's orbit around the Sun. This is the reason that a draconic month or nodal period (the period the Moon takes to return to the same node in its orbit) is shorter than the sidereal month. After one nodal precession period, the number of draconic months exceeds the number of sidereal months by exactly one. This period is about 6,793 days (18.60 years).[3]
As a result of this nodal precession, the time for the Sun to return to the same lunar node, the eclipse year, is about 18.6377 days shorter than a sidereal year. The number of solar orbits (years) during one lunar nodal precession period equals the period of orbit (one year[specify]) divided by this difference, minus one: 365.2422/18.6377 − 1.[citation needed]
The precession cycle affects the heights of tides. During half the cycle, the high and low tides are less extreme; in the other half of the cycle, they are amplified, with high tides greater than average and low tides lower than average.[4][5]
See also
[edit]References
[edit]- ^ Patrick Moore (1983). The Guinness Book of Astronomy Facts & Feats. p. 29.
In 1968 the north pole star of the Moon was Omega Draconis; by 1977 it was 36 Draconis. The south pole star is Delta Doradus.
- ^ "Re: Can precession occur in the opposite direction?". Retrieved 2021-10-20.
- ^ "Equinoxal crossover of the spring full moon". Universities Space Research Association. 2024-04-22. Retrieved 2024-10-29.
- ^ Greicius, Tony (7 July 2021). "Study Projects a Surge in Coastal Flooding, Starting in 2030s". NASA. Archived from the original on 2022-06-03. Retrieved 2021-07-15.
- ^ Peng, Dongju; Hill, Emma M.; Meltzner, Aron J.; Switzer, Adam D. (2019-01-10). "Tide Gauge Records Show That the 18.61-Year Nodal Tidal Cycle Can Change High Water Levels by up to 30 cm". Journal of Geophysical Research: Oceans. 124 (1): 736–749. Bibcode:2019JGRC..124..736P. doi:10.1029/2018JC014695. hdl:10220/49096. ISSN 2169-9275.
- Seidelmann, P.K., ed. (1992). Explanatory Supplement to the Astronomical Almanac. U.S. Naval Observatory / University Science Books. pp. 114–115, 701.
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Lunar precession
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Fundamentals
Definition and types
In celestial mechanics, precession refers to the gradual, torque-induced wobble or rotation of a rotating body's axis, resulting from gravitational interactions that produce uneven torques on the body's equatorial bulge or orbital plane. For the Moon, this phenomenon manifests in both its orbital orientation around Earth and its own rotational spin axis, driven primarily by the Sun's gravitational perturbations on the Earth-Moon system.[3] Lunar precession encompasses three main types: nodal precession, which involves the westward regression of the Moon's orbital nodes (the points where its orbit intersects the ecliptic plane) over approximately 18.6 years; apsidal precession, characterized by the eastward advance of the line of apsides (connecting perigee and apogee) with a period of about 8.85 years; and axial precession, the westward wobble of the Moon's rotational axis, also on an 18.6-year timescale and locked to the orbital nodal motion due to tidal synchronization.[1][5][6] These precessional motions are crucial for interpreting variations in the lengths of different lunar months (such as the draconic and anomalistic months), the timing and predictability of solar and lunar eclipses (which occur only near the nodes), long-term tidal patterns on Earth (influenced by changing perigee positions and declinations), and shifts in the Moon's polar orientations relative to the ecliptic (affecting visible polar regions from Earth).[7][1] In contrast to Earth's axial precession, which completes a full cycle in about 25,772 years due to combined solar and lunar torques on its oblate figure, lunar precession operates on much shorter timescales and is dominated by solar influences on the inclined, eccentric lunar orbit.[8]Historical background
Ancient Babylonian astronomers around 500 BCE inferred the existence of nodal precession through their analysis of the 18-year Saros cycle, which they used to predict the timing of solar and lunar eclipses based on patterns in observational records.[9][10] This empirical approach relied on compiling extensive eclipse data to identify recurring intervals, laying the groundwork for later understandings of lunar orbital dynamics.[11] In the 2nd century BCE, the Greek astronomer Hipparchus documented the apsidal precession of the Moon by observing the motion of its apogee, contributing key insights to early lunar theory.[12] Building on this, Ptolemy's Almagest around 150 CE incorporated an approximate model for apsidal motion within his geocentric framework, attempting to account for observed irregularities in the Moon's path.[13] During the Renaissance, Tycho Brahe and Johannes Kepler in the 16th and 17th centuries advanced these efforts with precise observational data on lunar orbital irregularities, refining models through meticulous telescopic measurements that highlighted deviations from simpler circular paths.[14] The 18th and 19th centuries marked a shift toward theoretical explanations, with Isaac Newton's Philosophiæ Naturalis Principia Mathematica (1687) providing a gravitational basis for lunar precessions by demonstrating how mutual attractions between celestial bodies induce such motions.[15] Pierre-Simon Laplace and contemporaries further developed this by applying perturbation theory to quantify the influences of solar and planetary gravity on the Moon's orbit.[16] In the modern era, 19th- and 20th-century telescopic observations confirmed details of axial precession, while astronomer Patrick Moore popularized the concept of changing lunar pole stars in his 1983 work, illustrating the long-term wobble of the Moon's rotational axis. Twenty-first-century advancements, including laser ranging and missions like the Lunar Reconnaissance Orbiter launched in 2009, have enabled high-precision refinements to these models through direct measurements of orbital parameters.[17] This progression reflects an evolution from empirical predictions rooted in eclipse cycles to comprehensive theoretical frameworks grounded in gravitational physics, with early gaps in axial precession understanding persisting until the full implications of tidal locking—where the Moon's rotation synchronizes with its orbit—were appreciated in the 19th century.[18] These three types of precession—nodal, apsidal, and axial—served as foundational elements in building increasingly accurate astronomical models over time.Nodal precession
Description and period
Nodal precession, also known as the regression of the lunar nodes, is the gradual rotation of the Moon's orbital plane around the ecliptic pole due to gravitational perturbations. The lunar nodes are the two points where the Moon's orbit intersects the ecliptic plane, with the ascending node being where the Moon crosses from south to north, and the descending node the opposite. This precession causes the nodes to regress westward along the ecliptic at a varying rate, viewed clockwise from the north ecliptic pole.[1] The period of nodal precession is approximately 18.6 years, more precisely 18.613 years or 6,793 days, during which the nodes complete one full revolution relative to the fixed stars. The speed of precession varies depending on the angular position of the nodes relative to the Sun, speeding up when aligned with the Sun and slowing otherwise. This motion is distinct from the Moon's axial precession, though the two are coupled due to tidal locking.[7][1]Causes and effects
The primary cause of nodal precession is the gravitational torque exerted by the Sun on the Earth-Moon system, arising from the 5.1° inclination of the Moon's orbital plane relative to the ecliptic. This torque acts to align the lunar orbit with the ecliptic but, due to the system's angular momentum, results in a precession of the orbital plane rather than a change in inclination. Contributions from other planets, such as Jupiter, introduce minor perturbations, but the Sun dominates.[1][3] The effects of nodal precession are significant for celestial observations and Earth's environment. It shifts the timing of solar and lunar eclipses over the 18.6-year cycle, as eclipses occur only when the Sun is near the nodes; the regression shortens the eclipse year to about 346.62 days compared to the tropical year of 365.24 days. Additionally, it produces the lunar standstill, where the Moon's maximum declination varies cyclically: reaching ±28.6° during major standstills (e.g., 2024–2025) and ±18.3° during minor standstills midway through the cycle, affecting the Moon's rising and setting positions and visibility from Earth.[1][3] On Earth, nodal precession modulates tidal amplitudes, with the 18.6-year cycle influencing sea level variations by up to 30 cm in some regions due to changes in the Moon's declination and distance. Combined with sea level rise from climate change, this can amplify coastal flooding risks, particularly in the mid-2030s when a minor standstill aligns with higher tides.[7][19]Apsidal precession
Description and period
Apsidal precession of the Moon's orbit is the gradual rotation of the line of apsides—the line connecting the points of closest approach (perigee) and farthest distance (apogee) from Earth—in the direction of the Moon's orbital motion. This precession occurs counterclockwise when viewed from the north ecliptic pole, completing one full cycle every 8.85 years, at a rate of approximately 40.7° per year relative to the fixed stars.[1][2] This motion makes the anomalistic month (time from perigee to perigee) slightly longer than the sidereal month (time relative to stars), by about 7.5 hours on average, due to the precession of the apsides. The precession interacts with the Moon's orbital eccentricity (currently about 0.0549), causing the positions of perigee and apogee to shift continuously.[1]Causes and effects
The primary cause of lunar apsidal precession is the gravitational perturbation from the Sun, which exerts a torque on the Moon's eccentric orbit around Earth. This solar influence distorts the lunar orbit, causing the apsides to advance in the prograde direction, opposite to the retrograde nodal precession. Contributions from other planets, such as Jupiter, and general relativistic effects add minor perturbations, but the Sun dominates, accounting for nearly all of the observed rate.[1][2] The effects of apsidal precession include variations in the timing of perigees and apogees relative to the Sun-Earth alignment, which influences tidal forces on Earth. For example, when perigee coincides with a new or full moon (syzygy), it produces perigean spring tides that are significantly higher than average, enhancing flood risks in coastal areas. Over the 8.85-year cycle, the frequency and intensity of these extreme tides modulate, with peaks occurring roughly every 4.425 years when perigee aligns toward or away from the Sun. Additionally, combined with nodal precession, apsidal motion contributes to long-term patterns in the visibility and characteristics of solar and lunar eclipses by altering the geometry of the Moon's path relative to the ecliptic.[1][2]Axial precession
Description and period
Axial precession of the Moon refers to the slow, retrograde circling of its north and south rotational poles in space, driven by gravitational torques from the Earth and Sun. The Moon's spin axis is tilted by approximately 1.54° relative to the normal to its orbital plane, resulting in a total inclination of about 6.68° with respect to the ecliptic plane.[20][21] This precession occurs over a period of 18.613 years, which matches the period of the Moon's nodal precession, at a rate of approximately -19.35° per year westward relative to the fixed stars. As a result, the rotational poles trace small circles on the celestial sphere with a radius of 1.54°.[22][20] Due to the Moon's tidal locking, which synchronizes its rotational and orbital periods, the spin axis remains nearly perpendicular to the orbital plane and precesses in tandem with the regression of the lunar nodes. The obliquity of the spin axis varies slightly by about 0.02° over the precession cycle.[21] The Moon maintains this configuration in a stable Cassini state, where the spin precession rate approximates the nodal precession rate .[20] As of 2025, the lunar north rotational pole lies in the constellation Draco, tracing its path around the north ecliptic pole, while the south rotational pole is situated in Dorado, near the bright star Canopus in the adjacent constellation Carina.[20]Causes and effects
The primary cause of the Moon's axial precession is the gravitational torque exerted by Earth's equatorial bulge on the Moon's permanent tidal bulge, which is fixed due to tidal locking.[20] This torque forces the Moon's spin axis to precess in synchrony with the orbital nodal motion over an 18.6-year period, maintaining a stable Cassini state where the spin axis, orbital pole, and ecliptic pole remain coplanar.[23] The Sun provides a secondary, minor torque on the Moon's ellipsoidal figure, contributing to the overall precessional dynamics but at a much lower magnitude than Earth's influence.[20] Additional factors include the Moon's equatorial ellipticity, which arises from its asymmetric figure, and dynamics within the inner core, leading to slight decoupling between the core and mantle.[23] This decoupling allows the inner core to undergo a forced precession at the 18.6-year period, with a tilt angle relative to the mantle on the order of degrees, influenced by the free inner core nutation frequency exceeding 2π/16.4 years⁻¹.[20] The mantle tilt in this equilibrium is approximately 1.543°, resulting from the balance of external torques and internal structure.[20] The precession cyclically alters the visibility of the Moon's polar regions from Earth, as the spin axis traces a small circle around the ecliptic pole, shifting the orientation of the poles relative to the celestial sphere over the 18.6-year cycle.[23] For instance, the star nearest the lunar north pole changes over the cycle, but the pole remains within the constellation Draco, which affects long-term observational access to polar features. Over a full cycle, this motion, combined with libration, enables observation of an additional small portion (~3.5%) of the lunar surface near the poles than would be possible without precession, enhancing opportunities for mapping and exploration.[24] Axial precession also modulates the amplitudes of optical librations by about 0.1°, introducing subtle variations in the apparent position of surface features and influencing detailed photometric and altimetric studies.[20] These changes impact the visibility of potential human exploration sites, particularly in polar regions, by periodically exposing shadowed craters to sunlight or Earth-based view.[23] On longer timescales, axial precession contributes to secular variations in the Moon's obliquity, driven by tidal evolution of the Earth-Moon system, where increasing orbital distance and slowing Earth rotation alter torque balances over millions of years.[25] This evolution can shift the obliquity from higher initial values toward the current stable 6.68° relative to the ecliptic, affecting the overall rotational stability and polar illumination patterns.[25]References
- https://www.coastalwiki.org/wiki/Long-period_lunar_tides