The Moon

Natural satellite of the Earth. Mass 7.35 x 10^25 g = 1/81 or 0.0123 Earth's. Mean radius 1740 km = ~ 1/4 the Earth's; this relatively small size ratio makes the Earth-Moon system unique in the solar system. Mean density 3.34 g cm-3. Mean distance from Earth 384,400 km. Escape velocity 2.38 km s-1. Surface gravity 162.2 cm s-2 = 0.165 Earth's. Sidereal period 27d 7h 43m 11s. Eccentricity 0.0549. Inclination of orbital plane to ecliptic 5° 8' 43''. Obliquity 6° 41'. Synodic period 29d 12h 44m 2s.9. Orbital velocity 1.02 km s-1. The Moon's average visual albedo is 0.12, a factor of three smaller than that of Earth. The Moon's center of mass is displaced about 2 km in the direction of Earth. The average temperature on the surface of the Moon during the day is 107 °C. During the night, the average temperature drops to -153 °C. Studies of lunar rock have shown that melting and separation must have begun at least 4.5 x 109 years ago, so the crust of the Moon was beginning to form a very short time after the solar system itself. Thickness of crust ~ 60 km; of mantle ~ 1000 km. Temperature of core ~ 1500 K. It would have taken only 107 years to slow the Moon's rotation into its present lock with its orbital period. Because of this synchronous rotation, the Moon revolves once on its axis each time it orbits the Earth, thus always presenting the same face, the nearside, toward Earth. The Moon may have formed during a collision between the early Earth and a Mars-sized rocky planet about 4.6 billion years ago; Theia.