![]() ![]() The Moon crosses the same node every 27.2122 days, an interval called the draconic month or draconitic month. The nodes are points at which the Moon's orbit crosses the ecliptic. It also means that when the moon is farthest north of the ecliptic, the centre of the part we see is about 6.7° south of the lunar equator and the south pole is visible, whereas when the moon is farthest south of the ecliptic the centre of the visible part is 6.7° north of the equator and the north pole is visible. Therefore, the angle between the ecliptic and the lunar equator is always 1.543°, even though the rotational axis of the Moon is not fixed with respect to the stars. As was discovered by Jacques Cassini in 1722, the rotational axis of the Moon precesses with the same rate as its orbital plane, but is 180° out of phase (see Cassini's Laws). The rotational axis of the Moon is not perpendicular to its orbital plane, so the lunar equator is not in the plane of its orbit, but is inclined to it by a constant value of 6.688° (this is the obliquity). If this had not happened, the Moon would now lie much closer to the ecliptic and eclipses would be much more frequent. It is thought that originally the inclination to the equator was near zero, but it could have been increased to 10° through the influence of planetesimals passing near the Moon while falling to the Earth. ![]() It would require an inclination of this earlier orbit of about 10° to the equator to produce a present inclination of 5° to the ecliptic. Theoretical considerations show that the present inclination relative to the ecliptic plane arose by tidal evolution from an earlier near-Earth orbit with a fairly constant inclination relative to Earth's equator. The mean inclination of the lunar orbit to the ecliptic plane is 5.145°. At such times, the earth's north pole is toward the moon and the moon is north of the ecliptic. This shows the specific configuration at major northern lunistice. Orbital inclination-the Moon's orbit is inclined by 5.14° to the ecliptic. The mean angular movement relative to an imaginary observer at the Earth–Moon barycentre is 13.176° per day to the east ( J2000.0 epoch). The variance in the Moon's orbital distance corresponds with changes in its tangential and angular speeds, as stated in Kepler's second law. The largest possible apparent diameter of the Moon is the same 12% larger (as perigee versus apogee distances) than the smallest the apparent area is 25% more and so is the amount of light it reflects toward Earth. An event referred to as a " supermoon" occurs when the full Moon is at its closest to Earth (perigee). Since nearer objects appear larger, the Moon's apparent size changes as it moves toward and away from an observer on Earth. The equation of the ellipse yields an eccentricity of 0.0549 and perigee and apogee distances of 362,600 km and 405,400 km respectively (a difference of 12%). The orbit of the Moon is a nearly circular ellipse about the Earth (the semimajor and semiminor axes are 384,400 km and 383,800 km, respectively: a difference of only 0.16%). Ĭomparison of the Moon's apparent size at lunar perigee– apogee. The Moon's orbit around Earth has many variations ( perturbations) due to the gravitational attraction of the Sun and planets, the study of which ( lunar theory) has a long history. The properties of the orbit described in this section are approximations. The Moon's orbital plane is inclined by about 5.1° with respect to the ecliptic plane, whereas the Moon's equatorial plane is tilted by only 1.5°. The Moon differs from most satellites of other planets in that its orbit is close to the ecliptic plane instead of to its primary's (in this case, Earth's) equatorial plane. With a mean orbital velocity of 1.022 km/s (0.635 miles/s, 2,286 miles/h), the Moon covers a distance approximately its diameter, or about half a degree on the celestial sphere, each hour. On average, the distance to the Moon is about 385,000 km (239,000 mi) from Earth's centre, which corresponds to about 60 Earth radii or 1.282 light-seconds. Earth and the Moon orbit about their barycentre (common centre of mass), which lies about 4,670 km (2,900 mi) from Earth's centre (about 73% of its radius), forming a satellite system called the Earth–Moon system. ![]() The Moon orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the stars in about 27.32 days (a tropical month and sidereal month) and one revolution relative to the Sun in about 29.53 days (a synodic month). While angles and relative sizes are to scale, distances are not. Diagram of the Moon's orbit with respect to the Earth. ![]()
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