The Moon

Updated February 12, 2020 | Infoplease Staff

Mercury and Venus do not have any moons. The planet that comes after Earth, Mars, has two very small moons. Jupiter has four major moons and at least 59 minor ones. Saturn, the ringed planet, has 56 known moons, of which 1 (Titan) is larger than the planet Mercury. Uranus has at least 27 moons (four of them large) as well as rings, while Neptune has 1 large and 12 small moons. Dwarf planet Pluto has three moons, one discovered in 1978 and two other moons first sighted by the Hubble Space Telescope in Oct. 2005.

Our Moon, with a diameter of 2,160 mi, is one of the larger moons in our solar system and is especially large when compared with the planet that it orbits. In fact, the common center of gravity of the Earth-Moon system is only about 1,000 mi below Earth's surface. The closest the Moon can come to us (its perigee) is 221,463 mi; the farthest it can go away (its apogee) is 252,710 mi. The period of rotation of the Moon is equal to its period of revolution around Earth, so from Earth we can see only one hemisphere of the Moon. Both periods are 27 days, 7 hours, 43 minutes, and 11.47 seconds. But while the rotation of the Moon is constant, its velocity in its orbit is not, since it moves more slowly in apogee than in perigee. Consequently, some portions near the rim of the Moon that are not normally visible will appear briefly. This phenomenon is called libration, and by taking advantage of the librations, astronomers have succeeded in mapping approximately 59% of the lunar surface. The other 41% can never be seen from Earth but has been mapped by American and Russian Moon-orbiting spacecraft.

Though the Moon goes around Earth in the time mentioned, the interval from new moon to new moon is 29 days, 12 hours, 44 minutes, and 2.78 seconds. This delay of nearly two days is due to the fact that Earth is moving around the Sun, so that the Moon needs two extra days to reach a spot in its orbit where no part is illuminated by the Sun, as seen from Earth.

If the plane of Earth's orbit around the Sun (the ecliptic) and the plane of the Moon's orbit around Earth were the same, the Moon would be eclipsed by Earth every time it is full, and the Sun would be eclipsed by the Moon every time the Moon is “new” (it would be better to call it the “black moon” when it is in this position). But because the two orbits do not coincide, the Moon's shadow normally misses Earth and Earth's shadow misses the Moon. The inclination of the two orbital planes to each other is 5°.

The tides are caused by the Moon with the help of the Sun, but in the open ocean they are surprisingly low, amounting to about one yard. The very high tides that can be observed near the shore in some places are due to funneling effects of the shorelines. At new moon and at full moon the tides raised by the Moon are reinforced by the Sun; these are the spring tides. If the Sun's tidal power acts at right angles to that of the Moon (quarter moons), we get the low neap tides.

The low-budget 1998 Lunar Prospector mission tentatively announced the presence of water ice on the Moon’s polar caps, but a controlled crash of the spacecraft turned up no physical evidence of water.

Plans are being made for the United States to return humans to the Moon by 2018. This time the aim is to eventually establish permanent outposts with crews staying up to six months on the lunar surface.

See also: Moons of the Solar System Slideshow

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