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A few facts….
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Sidereal period (completes a revolution in an elliptical (oval) orbit around Earth |
27 days, 7 hours, 43 mins and 11.5 seconds |
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Diameter |
3, 476 km / 2,160 miles |
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Orbital radius - max - min |
406,610 km / 252, 667 miles 356, 330 km / 221, 423 miles |
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Mean Orbital Velocity (speed) |
3, 680 km/h (2, 287 mph) |
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Surface Gravity |
1/6 Earth's 1.62m/s² (5.31 ft / sec²) |
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Surface Temperature Range |
+130ºC day, - 150ºC night |
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Average Density |
0.6 cm³/g |
This mosaic shows images of Earth and the Moon acquired by the Near Earth Asteroid Rendezvous Spacecraft (NEAR) on January 23, 1998.
The Moon is a natural satellite of the Earth, and the mass of the Earth is 81 times greater than that of the Moon. The Moon has no free water and absolutely no atmosphere, which means sound cannot travel and cannot be heard, and parachutes cannot be used for landing. The lack of atmosphere also means there is no weather to change its surface, though everything is coated with a fine layer of space-dust, but the Moon is not totally inactive.
As it orbits the Earth, the Moon keeps the same face towards us all the time, so we never actually see its back. The Soviet satellite, Luna 3, sent back the first photos of the far side, as it orbited the Moon in 1959. Nine years later, in 1968, three US astronauts of the Apollo 8 mission were the first astronauts to get a direct view of the back of the Moon. It is quite different from the side we see from Earth - more rugged, with many craters and not as many 'dark patches' as there are on the side we see from Earth.
As it moves along its orbit around the Earth, the Moon shows progressively evolving changes. These are known as the phases of the Moon. Half of the Moon is always in sunlight, just like half the Earth has day, while the other half has night. The phases of the Moon depend on how much of the sunlit half can be seen at any one time. When more than half the Moon can be seen, it is said to be gibbous. In the phase called the 'new moon', the face is completely in shadow. About a week later, the Moon is a luminous half-circle, at its first quarter; halfway through its cycle it shows a fully lighted surface, a week afterwards, at its last quarter, the Moon appears as a half- circle again.
The Moon is full when it is further away from the Sun than the Earth; it is new when it is closer. The Moon is said to be waxing when it evolves from new to full, and to be waning when it progresses again to new. This cycle is repeated every lunar month.
There are many theories concerning the formation of the Moon, and here are the four major ones:
Formation by Fission from the Earth - the Moon was spun-off from the Earth when the Earth was young and rotating rapidly on its axis.
Formation in Orbit Near the Earth - the Earth, Moon and all other bodies of the Solar System condensed, independently, out of the huge cloud of cold gases and solid particles that composed the earliest solar nebula (a cloud of gas or dust in space).
Formation Far from the Earth - independent formation of the Moon and the Earth (see previous theory), but the Moon formed in a different place in the Solar System. Their orbits then, by some means, carried them near each other, and the Moon was drawn into a permanent orbit around the Earth.
Planetesimal Impact - a large body called a planetesimal (about the size of Mars) struck the Earth, and portions of the Earth and the planetesimal were blasted off during the catastrophic impact into Earth orbit, eventually coalescing (merging) to form the Moon.
In 1975, having studied Moon rocks and close-up pictures of the Moon, scientists decided that the planetesimal theory was the most probable of the theories of formation.
Every one of the six manned landings on the Moon - Apollo missions 11, 12 and 14-17 - brought back samples of rock and soil, altogether weighing 384 kg (846 lb.).
Glossary
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This page of 'The Development of Space Travel and the First Moon Landing' was designed and created by E.Abrosimova. Copyright © 2000 North London Collegiate School. All rights reserved.