Our nearest neighbour in space, our Moon is the largest natural satellite of any planet compared with that planet. Although the Galilean moons around Jupiter are larger - so too is Jupiter - and the same applies to Titan around Saturn.
We naturally consider the Moon to orbit the Earth, but the orbit can also be interpreted as being around the Sun and intertwined with the Earth's orbit. Some people consider the Earth-Moon system as a double planet rather than as two separate entities.
The Moon formed when a Mars sized planetoid crashed into the Earth 4.5 billion years ago. It was not a head-on crash, and the impactor subsequently left the Solar System.
According to simulations carried out by Robin Canup et al from the University of Colorado, Boulder and reported in the 25 September 1997 edition of Nature, it took less than a year to form a solid body in Earth-orbit from the disc of debris thrown up by the collision. Out of 27 diffrent simulations one-third of them formed a two-moon system. Both objects would have survived for some time before merging into one body - the Moon we know today.
The Moon is still moving away from the Earth at a rate of 4cm per year.
| Equatorial Diameter | 3476 km |
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| Polar Diameter | 3476 km | |||||||||
| Ellipticity | 0.002 | |||||||||
| Mass | 7.3483 x 10 22 kg | |||||||||
| Volume (Earth=1) | 0.02 | |||||||||
| Density (water=1) | 3.34 | |||||||||
| Surface Gravity (Earth=1) | 0.165 | |||||||||
| Escape velocity | 2.37 kilometres / second | |||||||||
| Axial inclination | 1.53º | |||||||||
| Albedo | 0.07 | |||||||||
| David Harland's book Exploring The Moon provides a fascinating and unique insight into the science behind the Apollo expeditions.
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Mean Distance from Earth | 384.4 thousand kilometres | |
| Mean Sidereal Period | 27.322 Earth-days | ||
| Eccentricity | 0.055 | ||
| Inclination | 18.3º to 28.6º | ||
| Reciprocal Mass (Earth=1) | 81.3 | ||
| Opposition Magnitude (Average) | -12.7 | ||
The American spacecraft Lunar Prospector was launched on Jan. 6, 1998, from Cape Canaveral Air Station, aboard an Athena 2 rocket. In March 1998, mission scientists announced their first tentative findings of the presence of water ice in shadowed craters near the Moon's south and north poles. Scientists later estimated as much as six billion metric tons of water ice may be buried in these craters under about 18 inches of soil, in more concentrated deposits than originally thought. However, the evidence was indirect, they cautioned, based on reasonable scientific assumptions given the levels of hydrogen detected, from which water ice is inferred.
In a low-budget attempt to wring one last bit of scientific productivity from the low-cost Lunar Prospector mission, NASA worked with engineers and astronomers at the University of Texas to precisely crash the barrel-shaped spacecraft into a specific shadowed crater. NASA accepted the team's proposal based on successful scientific peer review of the idea and the pending end of the spacecraft's useful life, although the chances of positive detection of water were judged to be less than 10 percent.
Worldwide observations of the crash on 31 July 1999 were focused primarily on using sensitive spectrometers tuned to look for the ultraviolet emission lines expected from the hydroxyl (OH) molecules that should be a by-product of any icy rock and dust kicked up by the impact of the 354-pound spacecraft.
"There are several possible explanations why we did not detect any water signature, and none of them can really be discounted at this time," said Dr. Ed Barker, assistant director of the University of Texus' McDonald Observatory at Austin, who co-ordinated the observing campaign.
These explanations include:
I get very annoyed when people spell Moon with a small "m". The Moon is a place, like London or Chicargo. It can also be a type of celestial object - a natural satellite of a planet - thus it can be written that Europa is a moon of Jupiter. Europa and Jupiter are always given capital letters - so why do people insist on writing "earth" and "moon"?
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