Lunar precession

{{main|Axial precession}}

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,{{cite book |title=The Guinness Book of Astronomy Facts & Feats |page=29 |author=Patrick Moore |author-link=Patrick Moore |year=1983 |quote=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.}} 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 {{cite web|access-date=2021-10-20|title=Re: Can precession occur in the opposite direction?|url=http://www.madsci.org/posts/archives/2001-09/1001187215.As.r.html}} - whereas Apsidal precession is in the same direction as the rotation (meaning apsidal precession is eastward).

{{main|Apsidal precession}}

File:Moon apsidal precession.png occurs when the direction of the major axis of the Moon's elliptic orbit rotates once every 8.85 years in the same direction as the Moon's rotation itself. This image looks upwards depicting Earth's geographic south pole and the elliptical shape of the Moon's orbit (which is vastly exaggerated from its almost circular shape to make the precession evident) is rotating from white to greyer orbits.]]

{{multiple image|perrow = 1|total_width=230

| image1 = Animation of Moon orbit around Earth - Polar view.gif

| image2 = Animation of Moon orbit around Earth - Equatorial view.gif

| footer = Animation of Moon orbit around Earth
{{legend2|magenta|Moon}}{{·}}{{legend2|RoyalBlue|Earth}}
Top: Polar view; Bottom: Equatorial view}}

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).

File:Eclipse vs new or full moons, annotated.svg of the Moon's orbit results in relative revolution of the lunar nodes as the Earth revolves around the Sun. This causes an eclipse season approximately every six months. Nodal precession occurs every 18.6 years.]]

File:Lunar eclipse diagram-en.svg are the points where the Moon's orbit intersects the ecliptic.]]

{{main|Nodal precession}}

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).{{Cite web |date=2024-04-22 |title=Equinoxal crossover of the spring full moon |url=https://epod.usra.edu/blog/2024/04/equinoxal-crossover-of-the-spring-full-moon.html |access-date=2024-10-29 |website=Universities Space Research Association}}

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 |reason=Sidereal? Synodic? Seems tropical was used but no source given. |date=October 2022}}) divided by this difference, minus one: {{sfrac|365.2422|18.6377}} − 1.{{fact|date=October 2022}}

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.{{cite web |last1=Greicius |first1=Tony |date=7 July 2021 |title=Study Projects a Surge in Coastal Flooding, Starting in 2030s |url=http://www.nasa.gov/feature/jpl/study-projects-a-surge-in-coastal-flooding-starting-in-2030s |access-date=2021-07-15 |website=NASA |archive-date=2022-06-03 |archive-url=https://web.archive.org/web/20220603031628/https://www.nasa.gov/feature/jpl/study-projects-a-surge-in-coastal-flooding-starting-in-2030s/ |url-status=dead }}{{Cite journal |last1=Peng |first1=Dongju |last2=Hill |first2=Emma M. |last3=Meltzner |first3=Aron J. |last4=Switzer |first4=Adam D. |date=2019-01-10 |title=Tide Gauge Records Show That the 18.61-Year Nodal Tidal Cycle Can Change High Water Levels by up to 30 cm |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JC014695 |journal=Journal of Geophysical Research: Oceans |language=en |volume=124 |issue=1 |pages=736–749 |doi=10.1029/2018JC014695 |bibcode=2019JGRC..124..736P |issn=2169-9275|hdl=10220/49096 |hdl-access=free }}

• Lunar theory
• Orbit of the Moon

{{reflist}}

• {{cite book |editor=Seidelmann, P.K. |year=1992 |title=Explanatory Supplement to the Astronomical Almanac |publisher=U.S. Naval Observatory / University Science Books |pages=114–115, 701}}

{{The Moon}}

{{DEFAULTSORT:Lunar Precession}}

Category:Orbit of the Moon