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Lunar precession is a term used for three different precession motions related to the
This kind of precession is that of the major axis of the Moon's elliptic orbit (the line of the
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
Lunar precession is a term used for three different precession motions related to the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
.
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 important types of precessional motion: apsidal and nodal.
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, the lunar north pole describes a small circle around a point in the constellationDraco
Draco is the Latin word for serpent or dragon.
Draco or Drako may also refer to:
People
* Draco (lawgiver) (from Greek: Δράκων; 7th century BC), the first lawgiver of ancient Athens, Greece, from whom the term ''draconian'' is derived
* ...
, 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 - whereas Apsidal precession is in the same direction as the rotation (meaning apsidal precession is eastward).
Apsidal precession
This kind of precession is that of the major axis of the Moon's elliptic orbit (the line of the apsides
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
from perigee
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any el ...
to apogee
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any el ...
), which precesses eastward by 360° in approximately 8.85 years. This is the reason that an anomalistic month
In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.
Variations
In Shona, Middle Eastern, and Euro ...
(the period the Moon moves from the perigee to the apogee and to the perigee again) is longer than the sidereal month
In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.
Variations
In Shona, Middle Eastern, and ...
(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
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
In general, a node is a localized swelling (a " knot") or a point of intersection (a vertex).
Node may refer to:
In mathematics
* Vertex (graph theory), a vertex in a mathematical graph
* Vertex (geometry), a point where two or more curves, line ...
to move through 360° relative to the vernal equinox (autumnal equinox in 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 if seen from the celestial north. 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).
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) divided by this difference, minus one: − 1.
The precession cycle affects the heights of tides. During half the cycles 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.
See also
*Lunar theory Lunar theory attempts to account for the motions of the Moon. There are many small variations (or perturbations) in the Moon's motion, and many attempts have been made to account for them. After centuries of being problematic, lunar motion can now ...
* Orbit of the Moon
References
* {{DEFAULTSORT:Lunar Precession Orbit of the Moon