Difference between revisions of "Moon"
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The '''Moon''' is Earth's satellite and orbits it<ref>It is more correct to say that the Earth and the Moon orbit each other around a gravitational midpoint called the ''barycenter.'' This point is, however, actually inside the Earth, about 3/4 from its center. It is not the Earth, but the barycenter of the Earth/Moon system, that orbits the Sun in a near-perfect ellipse. Relative to this ellipse, Earth is 3,000 miles closer to the Sun at full moon and 3,000 miles further from the Sun at new moon.</ref> at an average distance of about 238,000 miles (380,000 km.) | The '''Moon''' is Earth's satellite and orbits it<ref>It is more correct to say that the Earth and the Moon orbit each other around a gravitational midpoint called the ''barycenter.'' This point is, however, actually inside the Earth, about 3/4 from its center. It is not the Earth, but the barycenter of the Earth/Moon system, that orbits the Sun in a near-perfect ellipse. Relative to this ellipse, Earth is 3,000 miles closer to the Sun at full moon and 3,000 miles further from the Sun at new moon.</ref> at an average distance of about 238,000 miles (380,000 km.) | ||
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| + | The gravitational force of the Moon causes the ocean [[tides]]; these tidal forces have other effects as well. | ||
Unlike most planets, Earth has only one moon and it has several striking characteristics: | Unlike most planets, Earth has only one moon and it has several striking characteristics: | ||
Revision as of 12:59, March 19, 2007
The Moon is Earth's satellite and orbits it[1] at an average distance of about 238,000 miles (380,000 km.)
The gravitational force of the Moon causes the ocean tides; these tidal forces have other effects as well.
Unlike most planets, Earth has only one moon and it has several striking characteristics:
1. Throughout man's existence, the Moon has had the same size as the Sun when viewed from Earth. This creates a beautiful symmetry and permits phenomenal eclipses to occur. The odds of that symmetry occurring by chance are very small. That symmetry will not last forever, as the sun is continually shrinking.
2. The Moon presents the same side to Earth at all times, even though the Moon revolves around the Earth. That requires the rotation of the Moon to be timed precisely to offset the separate effects of the revolution around Earth. Physicists explain this phenomenon by invoking tidal locking; tidal forces on Earth caused a bulge on the Moon, thereby creating a drag on the rotation of the Moon until its same face always presents to the Earth. It was once believed that the planet Mercury had a similar locked rotation due to gravitational forces from the sun, such that the same side of Mercury always faces the sun. In fact, Mercury's tidal locking is incomplete; it does not always present the same face to the sun, but rather switches the side that it presents to the sun every full revolution.
Current tidal forces exerted by the Earth on the Moon cannot account fully for its bulge or egg shape, though accounting for greater tidal forces in the past may explain this discrepancy.[2]
3. The Moon's surface lacks the abundant iron that permeates the Earth, thereby proving that the Moon did not come from the Earth. This deficiency of iron on the Moon disproved the theory that the Moon must have originated by breaking off from the Earth.
4. The Moon is currently receding from the Earth at less than 6 inches per year. The Moon could never have been closer than about 11,000 miles or it would have been broken up by tidal forces. If the rate of recession is assumed to have averaged about 6 inches per year, the Moon's present distance of about 250,000 miles implies an age for the Moon of no more than 2.6 billion years. However, the tidal forces between Earth and Moon are not constant, and so simply extrapolating from the current rate of change is not strictly valid.
See also
Notes and References
- ↑ It is more correct to say that the Earth and the Moon orbit each other around a gravitational midpoint called the barycenter. This point is, however, actually inside the Earth, about 3/4 from its center. It is not the Earth, but the barycenter of the Earth/Moon system, that orbits the Sun in a near-perfect ellipse. Relative to this ellipse, Earth is 3,000 miles closer to the Sun at full moon and 3,000 miles further from the Sun at new moon.
- ↑ http://physics.fortlewis.edu/Astronomy/astronomy%20today/CHAISSON/AT308/HTML/AT30803.HTM