Moon, natural satellite of a planet (see satellite,
natural), in particular, the single natural satellite of the earth.
1. The earth's natural satellite, orbiting the earth at a mean distance of 238,857 miles (384,393 km) and having a diameter of 2160 miles (3476 km).
2. This body during a particular lunar month, or during a certain period of time, or at a certain point of time, regarded as a
distinct object or entity. Cf. full moon, half-moon, new moon, waning moon, waxing moon.
3. A lunar month, or, in general, a month.
4. Any planetary satellite: the moons of Jupiter.
5. Something shaped like an orb or a crescent.
6. Moonlight.
7. A platy fish.
8. Slang.the buttocks, esp. when bared.
9. Blue moon, a very long period of time: Such a chance comes once in a blue moon.
M
Mercury and
Venus do not have any moons. The planet that comes after the
Earth, Mars, has two very small moons.
Jupiter has four major moons and at least 24 minor ones.
Saturn, the ringed planet, has 30 known moons, of which one (Titan) is larger than the planet
Mercury. Uranus has at least 21 moons (four of them large) as well as rings, while
Neptune has one large and seven small moons. Pluto has one moon, discovered in 1978. Some astronomers still consider Pluto to be a “runaway moon” of Neptune.
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 Lunar Prospector spacecraft, launched in Jan. 1998, found that as much as three billion metric tons of water ice is hidden in the permanently shaded craters at the poles. The water probably came from interstellar comets that crashed into the Moon. Lunar Prospector also confirmed that the Moon has a small core, supporting the theory that the Moon was ripped away from the early Earth when an object the size of Mars collided with the Earth.
At the end of its mission, on July 31, 1999, the spacecraft was intentionally crashed into a permanently shadowed crater at the Moon's south pole in the hope of detecting a rising plume of water ice, but no cloud of water vapor molecules was observed by powerful Earth telescopes.
The moon is the earth's nearest neighbor in space. In addition to its proximity, the moon is also exceptional in that it is quite massive compared to the earth itself, the ratio of their masses being far larger than the similar ratios of other natural satellites to the planets they orbit (with the exception of Charon and Pluto). For this reason, the earth-moon system is sometimes considered a double planet. It is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun in accordance with Kepler's laws. It is also more accurate to say that the earth and moon together revolve about their common center of mass, rather than saying that the moon revolves about the earth. This common center of mass lies beneath the earth's surface, about 3,000 mi (4800 km) from the earth's center.
The Lunar Month
The moon was studied, and its apparent motions through the sky recorded, beginning in ancient times. The Babylonians and the Maya, for example, had remarkably precise calendars for eclipses and other astronomical events. Astronomers now recognize different kinds of months, such as the synodic month of 29 days, 12 hr, 44 min, the period of the lunar phases, and the sidereal month of 27 days, 7 hr, 43 min, the period of lunar revolution around the earth.
The Lunar Orbit
As seen from above the earth's north pole, the moon moves in a counterclockwise direction with an average orbital speed of about 0.6 mi/sec (1 km/sec). Because the lunar orbit is elliptical, the distance between the earth and the moon varies periodically as the moon revolves in its orbit. At perigee, when the moon is nearest the earth, the distance is about 227,000 mi (365,000 km); at apogee, when the moon is farthest from the earth, the distance is about 254,000 mi (409,000 km). The average distance is about 240,000 mi (385,000 km), or about 60 times the radius of the earth itself. The plane of the moon's orbit is tilted, or inclined, at an angle of about 5° with respect to the ecliptic. The line dividing the bright and dark portions of the moon is called the terminator.
Retarded Lunar Motion
Due to the earth's rotation, the moon appears to rise in the east and set in the west, like all other heavenly bodies; however, the moon's own orbital motion carries it eastward against the stars. This apparent motion is much more rapid than the similar motion of the sun. Hence the moon appears to overtake the sun and rises on an average of 50 minutes later each night. There are many variations in this retardation according to latitude and time of year. In much of the Northern Hemisphere, at the autumnal equinox, the harvest moon occurs; moonrise and sunset nearly coincide for several days around full moon. The next succeeding full moon, called the hunter's moon, also shows this coincidence.
Solar and Lunar Eclipses
Although an optical illusion causes the moon to appear larger when it is near the horizon than when it is near the zenith, the true angular size of the moon's diameter is about 1/2°, which also happens to be the sun's apparent diameter. This coincidence makes possible total eclipses of the sun in which the solar disk is exactly covered by the disk of the moon. An eclipse of the moon occurs when the earth's shadow falls onto the moon, temporarily blocking the sunlight that causes the moon to shine. Eclipses can occur only when the moon, sun, and earth are arranged along a straight line—lunar eclipses at full moon and solar eclipses at new moon.
Tidal Influence of the Moon
The gravitational influence of the moon is chiefly responsible for the tides of the earth's oceans, the twice-daily rise and fall of sea level. The ocean tides are caused by the flow of water toward the two points on the earth's surface that are instantaneously directly beneath the moon and directly opposite the moon. Because of frictional drag, the earth's rotation carries the two tidal bulges slightly forward of the line connecting earth and moon. The resulting torque slows the earth's rotation while increasing the moon's orbital velocity. As a result, the day is getting longer and the moon is moving farther away from the earth. The moon also raises much smaller tides in the solid crust of the earth, deforming its shape. The tidal influence of the earth on the moon was responsible for making the moon's periods of rotation and revolution equal, so that the same side of the moon always faces earth.
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