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Did you know...
...that the Moon is moving away from Earth at approximately 3 cm per year?
The Moon's gravity is only 1/6 that of Earth?
| radius: | | 1740 km |
| mass: | 7.35×1022 kg | ||
| mean distance from Earth: | 3.84×105 km |
Earth's Moon is the sixth largest satellite in the solar system (behind Jupiter's Io, Ganymede, and Europa, Saturn's Titan and Neptune's Triton). However, the five larger moons orbit gigantic planets several hundreds of times more massive than the moons themselves. The combined system of Earth and Moon is sometimes described as a double planet because the mass ratio of the two (80 to 1) is much less than for other planetary systems.
The footage of the Moon walks by astronauts remind us that gravity is much weaker on the Moon than on Earth. This weaker gravity doesn't hold gasses very tightly, so they can escape out into space very easily. This is one of the reasons that the Moon has no appreciable atmosphere. The Moon's atmosphere is 10-14 times as dense as Earth's, making it almost a total vacuum.
The Moon's lack of atmosphere has profound effects on its surface. With no atmosphere, harsh climatic conditions do not exist on the Moon. Its surface isn't subject to storms, floods, fires, or other landscape-altering events seen on Earth. There is also no free water on the Moon, so no erosion occurs. The only way the Moon's surface is changed is by outside forces--fragments of rock and meteorites which impact on the surface.
The relationship between Earth and the Moon is more complicated than it appears. The Sun pulls on the Moon much harder than Earth does. From other planets, the Moon would appear to orbit the Sun. The Sun will never pull the Moon away from the Earth, though; it pulls the two almost equally.
The Moon may appear bright in the sky, but it produces no light of its own. All of the Moon's brightness is merely the reflection of the Sun's light. The Moon's apparent shape, or "phase," depends on the position of the Moon with respect to the Sun and Earth. When the Sun, Earth, and Moon are all lined up in a row, an eclipse occurs. There are two types of eclipses: solar and lunar. A solar eclipse occurs when the Moon lies between the Sun and Earth, so that the Sun's light is blocked out. A lunar eclipse is when the Moon goes behind the Earth where the Sun's light is partially blocked, so the Moon seems to darken.
The Moon's gravitational pull on the Earth causes tides in Earth' oceans. The Moon's gravity actually pulls the water with it as it orbits. The Moon doesn't drag the continents around, of course, since rock is more resistant to motion. High tides occur near the part of the Earth nearest to the Moon, and on the opposite. Since there are two areas of high tide on the Earth, and since the Earth revolves on its axis once per day, each region of the Earth hits two high tides per day. Friction in Earth's oceans and the law of conservation of angular motion causes the size of Moon's orbit to increase. Very slowly, the Moon is pulling away from the Earth.
As is common among satellites, the Moon's day is equal to its year. This means that the same side of the Moon always faces Earth. In fact, no one knew what the "dark side" of the Moon looked like until 1959, it was photographed by the Luna 3 probe. The two sides are actually quite different. The near side is covered with impact craters called maria which have been filled with lava. Puzzlingly, none of these maria exist on the far side. There has even been evidence that water may be frozen inside some of the far side's craters, but no proof has been found yet.
There are several theories about where the Moon originated. One states that the Moon formed at the same time the Earth did. Another suggests that when Earth was still forming, it was rotating too fast to stay together, and part of it fell off and formed the Moon. A popular recent theory is that the Moon formed when some huge object impacted with the Earth. Evidence to support this theory includes the fact that both Earth and Moon are made of almost the exact same materials.
 | a man on the Moon |
* Photo credit - NSSDC Planetary Image Catalog
