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The orbit of the Moon around the Earth is completed in approximately 27.3 days. The Earth and Moon orbit about their common center of mass, which lies about 4,700 kilometres from Earth's center (about three quarters of the Earth's radius). On average, the Moon is at a distance of about 385,000 km from the center of the Earth, which corresponds to about 60 Earth radii. With a mean orbital speed of 1.023 km/s, the Moon moves relative to the stars each hour 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 to the Earth's equatorial plane. The lunar orbit plane is inclined to the ecliptic by 5.1°, whereas the Moon's spin axis is inclined by only 1.5°. Because of the relatively large size of the Moon and the relatively small Earth–Moon mass ratio of about 811 (when compared to other natural satellites in our solar system), some consider the Earth–Moon system to be a double planet. Based on the informal definition that a double planet system must have its barycenter exterior to both bodies, however, others regard the Earth and Moon as an ordinary planet–moon system. The orbit of the Moon is distinctly elliptical with an average eccentricity of 0.0549. The non-circular form of the lunar orbit causes variations in the Moon's angular speed and apparent size as it moves towards and away from an observer on Earth. The mean angular daily movement relative to an imaginary observer at the barycenter is 13.176358° to the east. The orientation of the orbit is not fixed in space, but precesses over time. One motion is the precession of the line of apsides the ellipse of the lunar orbit slowly rotates counterclockwise, and completes a full revolution in about 8.850 years (3,233 days). The other motion is associated with the (clockwise) precession of the orbital plane itself about an axis perpendicular to the ecliptic. The points where the lunar orbit intersects the ecliptic (the nodes) precess with time, completing one revolution in about 18.6 years (6,793 days). The mean inclination of the lunar orbit to the ecliptic plane is 5.145°. The rotation axis of the Moon is also 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). One might be tempted to think that as a result of the precession of the Moon's orbit plane, the angle between the lunar equator and the ecliptic would vary between the sum (11.833°) and difference (1.543°) of these two angles. However, as was discovered by Jacques Cassini in 1721, the rotation axis of the Moon precesses with the same rate as its orbit plane, but is 180° out of phase (see Cassini's Laws). Thus, although the rotation axis of the Moon is not fixed with respect to the stars, the angle between the ecliptic and the lunar equator is always 1.543°.
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