obliquity (inclination of equator to orbit)
orbital velocity (average) average distance from the Earth perigee aphelion orbital eccentricity orbital inclination to the ecliptic orbital period around the Earth rotation on its axis rotation speed at equator rotation direction
27.32166 Earth days
10.34 miles per hour (16.65 km/hr)
prograde (counterclockwise when viewed from above the North Pole)
27.32166 Earth days (rotates in the same amount of time it takes to orbit the Earth;
this is called synchronous rotation)
0.636 miles per second (1.023 km/sec)
238,862 miles (384,401 km)
225,700 miles (363,300 km)
252,000 miles (405,500 km)
5.145 degrees (oscillates 0.15 degrees in a 173-day cycle) 6.68 degrees
Humankind did not see the far side of the Moon until the launch of the Soviet space mission Luna 3 in October 1959. This mission flew around the back of the Moon and photographed its mysterious far side, finding an equally old and cratered surface, but one largely lacking the giant dark pools of mare basalts. This problem is still an active one in the lunar scientific community:Why does only the front of the Moon carry large volcanic surface flows? Theories include larger amounts of heat-producing radiogenic elements in the Moon's near-side mantle, a thicker crust that inhibits eruptions on the far side, and a preferential eruption pattern inside the Moon that happens to face toward the Earth. All these possibilities are tied to processes described in the section on the formation of the Moon, in chapter 7.
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