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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.
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