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The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.

The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector.

The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been reinforced by data from Lunar Prospector. There is apparently ice at the north pole as well. A final determination will probably come from NASA's Lunar Reconnaissance Orbiter, scheduled for 2008.

A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these precious samples.

Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion which asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large object (as big as Mars or more) and that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.
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