Fundamental Facts About The Moon

equatorial radius polar radius ellipticity ([equatorial radius polar radius]/polar radius) volume

1,079.6 miles (1,737.4 km), or 0.25 Earth's 1,077.4 miles (1,733.93 km)


average density acceleration of gravity on the surface at the equator magnetic field strength at the surface mass

5.267 X I09 cubic miles (2.197 X I010 km3), or 0.02 times Earth's

1.6186 X I023 pounds (7.3483 X I022 kg); just over I percent Earth's mass 210 pounds per cubic foot (3,34I kg/m3) 5.32 feet per square seconds (I.62 m/sec2)

varies from 6 to 313 X 10 9 T (about 1,000 times weaker than Earth's)

Solar and lunar eclipses are caused by the interactions of the shadows of the Earth and Moon. When the Moon is between the Earth and Sun, it can eclipse the Sun from a small portion of the Earth's surface; when the Earth's shadow crosses the Moon, it can cause a lunar eclipse.

between the bodies, the tidal bulges move around the rotating body to stay in alignment with the gravitational force between the bodies. This is why ocean tides on Earth rise and fall with the rising and setting of its Moon; the same effect occurs to some extent on all rotating orbiting bodies.

Since the bulge requires a small amount of time to shift position, the tidal bulge of the Moon is always located at a slight angle to the line between the closest points of the Moon and Earth. The misalignment of the tidal bulge with the body that caused it results in a small but significant gravitational force on the bulge, acting in the opposite direction of its rotation.The rotation of the satellite slowly decreases (and its orbital momentum simultaneously increases). This is the case where the Moon's rotational period is faster than its orbital period around its planet. If the opposite is true, tidal forces increase its rate of rotation and decrease its orbital momentum.

Almost all moons in the solar system are tidally locked with their primaries, since they orbit closely and tidal force strengthens rapidly with decreasing distance. In addition, Mercury is tidally locked with the Sun in a 3:2 resonance. Mercury is the only solar system body in a 3:2 resonance with the Sun. For every two times Mercury revolves around the Sun, it rotates on its own axis three times. In a more subtle way the planet Venus is tidally locked with the planet Earth, so that whenever the two are at their closest approach to each other in their orbits Venus always has the same face toward Earth (the tidal forces involved in this lock are extremely small). In general any object that orbits another massive object closely for long periods is likely to be tidally locked to it.

The Moon's tidal lock to the Earth made its far side even more mysterious: What could be there, on the unseen face of the Moon?

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