The Moon's orbit
The
Moon makes a complete orbit about the Earth approximately once every 27.3 days. Each hour, the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5?. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the
ecliptic and not in the Earth's
equatorial plane.
Encyclopedia
The
Moon makes a complete orbit about the Earth approximately once every 27.3 days. Each hour, the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the
ecliptic and not in the Earth's
equatorial plane.
Sidereal and Synodic
Several ways to consider a complete orbit are detailed in the table below, but the two most familiar are: the sidereal month being the time it takes to make a complete orbit with respect to the stars, about 27.3 days; and the synodic month being the time it takes to reach the same
phase, about 29.5 days. These differ because in the meantime the Earth and Moon have both orbited some distance around the
Sun.
Tides on earth
The
gravitational attraction that the Moon exerts on Earth is the cause of
tides in the sea. The tidal flow period, but not the phase, is synchronized to the Moon's orbit around Earth. The tidal bulges on Earth, caused by the Moon's gravity, are carried ahead of the apparent position of the Moon by the Earth's rotation, in part because of the friction of the water as it slides over the ocean bottom and into or out of bays and
estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 millimetres per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. A more detailed discussion follows in the section titled
Earth & Moon.
Synchronous rotation
The Moon is in synchronous rotation, meaning that it keeps the same face turned to the Earth at all times. This synchronous rotation is only true on average because the Moon's orbit has definite eccentricity. When the Moon is at its
perigee, its rotation is slower than its orbital motion, and this allows us to see up to an extra eight degrees of longitude of its East side. Conversely, when the Moon reaches its
apogee, its rotation is faster than its orbital motion and reveals another eight degrees of longitude of its West side. This is called
longitudinal libration.
Oscillate
Because the lunar orbit is also inclined to the Earth's
equator, the Moon seems to oscillate up and down as it moves in celestial latitude . This is called
latitudinal libration and reveals the Moon's polar zones over almost seven degrees of latitude. Finally, because the Moon is only at about 60 Earth radii distance, an observer at the equator who observes the Moon throughout the night moves by an Earth diameter sideways. This is the
diurnal libration and reveals about one degree's worth of lunar longitude. Observers at either geographical pole of the Earth can add one degree worth of libration in latitude.
Common center of mass
Earth and Moon orbit about their
barycenter, or common
center of mass, which lies about 4,700 kilometres from Earth's center . Since the barycenter is located below the Earth's surface, Earth's motion is more commonly described as a "wobble". When viewed from Earth's North Pole, Earth and Moon rotate counter-clockwise about their axes; the Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise.
Inclination of the lunar orbit
It may seem strange that the
inclination of the lunar orbit is listed as varying considerably. This, however, is only apparent because the angle is measured in
equatorial coordinates. If it were given in ecliptic coordinates it would be more stable.
The inclination of the lunar orbit on the
ecliptic has a mean value of 5.1453964°. However, the Sun distorts the shape of the Moon's 'elliptical' orbit so much that its inclination varies from 4.97° at syzygy to 5.32° at quadrature . The position of the ascending node steadily regresses , making a full circle in 18.6 years. The longitude where the moon reaches its maximum or minimum latitude therefore moves around too. Sometimes it is on a position where the ecliptic also reaches its maximum or minimum
declination of 23.4°. Sometimes , the two are just on opposite sides. When they coincide the values add up and the Moon can reach a maximum declination of 28.5° north or south. When the two are working against each other, they subtract leaving a declination range of only 18.3° north or south. For more details see
lunar standstill.
The points where the Moon's orbit crosses the ecliptic are called the
lunar nodes: the ascending node is where the Moon crosses to the north of the ecliptic; the descending node where it crosses to the south. Solar
eclipses occur when a node coincides with the
new Moon; lunar eclipses when a node coincides with the
full Moon.
Inclination of the rotation axis
The rotation axis of the Moon is not perpendicular to its orbital plane. The inclination between the lunar equator and the lunar orbit is a constant value of 1.5424°. One would be tempted to think that due to the nodal regression the angle between the lunar equator and the ecliptic would vary between the sum of the two angles and the difference . However, as already discovered by Jacques Cassini in 1721, the rotation axis of the Moon is tidally locked to the ecliptic such that its equatorial plane also regresses with the same speed as the orbit but 180° out of phase. Said otherwise the ascending node of the lunar orbit on the ecliptic always coincides with the descending node of the lunar equator on its orbit. Therefore the inclination of the Moon's equator on the ecliptic is always the same maximum value of 6.69°, which is the angle of libration in latitude as mentioned above. Also therefore the rotation axis of the Moon is not fixed among the stars.
The Earth & Moon's path around the Sun
In representations of the Solar system, it is common to draw the trajectory of the Earth from the point of view of the Sun, and the trajectory of the Moon from the point of view of the Earth, in a way that may suggest that the trajectory of the Moon circles around the Earth in such a way that sometimes it goes backwards. In fact, this never occurs. Unlike most other moons in the Solar System, the annual trajectory of the Moon is very similar to the one of the Earth and is always curved in the same way, concave towards the Sun, and nowhere convex or even looped towards the Sun.
The Moon is attracted towards the Sun more than it is towards the Earth, and the combined Sun-Earth "pull" that determines the path of the Moon is always directed inwards, towards the Sun.
The Moon's periods
Name | Value | Definition | | sidereal | 27.321 66155 | With respect to the distant stars |
| synodic | 29.530 588 | With respect to the Sun |
| tropical | 27.321 582 | With respect to the vernal point |
| anomalistic | 27.554 550 | With respect to the perigee |
| draconitic | 27.212 220 | With respect to the ascending node |
Other properties of the Moon's orbit
| Name | Value |
|---|
| Metonic cycle | 19×365 d |
| Semi-major axis | ~384 403 km |
| Distance at perigee | ~364 397 km |
| Distance at apogee | ~406 731 km |
| Mean eccentricity | 0.0549003
|
| Period of precession of nodes | 18.5996 a |
| Period of recession of line of apsides | 8.8504 a |
| Eclipse year | 346.621 d |
| Saros cycle | 18.030 a |
| Mean inclination of orbit to ecliptic | 5° 9' |
| Mean inclination of lunar equator to ecliptic | 1° 32' |
References
