Earth has only one moon. The distance from Earth to the Moon is approximately 384,000 km (238,607 miles). The Moon's diameter is 2,160 miles (3,476 km) and its mass and density is much less than Earth's.
The Moon is thought to have formed about 4.6 billion years ago. There are three theories as to how the Moon was formed. One is that it formed in much the same way and about the same time as Earth did. However, this doesn't seem too plausible because the Moon and the Earth are too different in composition from one another to have been formed at the same time and in the same general area of space.
Another theory is that the Moon formed somewhere else, but was pulled into Earth's orbit by Earth's gravitational field. This would explain why the composition of the Earth and Moon is so different. However, scientists think that it would have been very difficult for Earth to have pulled in a large satellite like the Moon.
The leading theory scientists have on the Moon's origin is the Impact Theory. This theory postulates that a very large object hit Earth's surface and knocked a bunch of rock from Earth's mantle into outerspace as hot vapor. The vaporous rock combined, cooled, and remained together and formed our Moon. Because the Moon formed from Earth's mantle and not from the core, this helps to explain the differences in the density and composition between Earth and the Moon. Also, the Moon doesn't seem to have a magnetic field like Earth does. As far as we know, the Moon only has magnetic fields in a few isolated locations. Also, we don't believe that the Moon has an iron core as Earth does, which would make sense if the Moon formed out of Earth's mantle.
The Moon has no real atmosphere, only trace amounts of a few gases. It is a barren, desolate place, with no oxygen, that cannot support life (as we know it). Because the Moon has no real atmosphere, it has wide temperature swings. During the day, the Moon's temperature reaches over 200 °F (93 °C), but at night the temperature plummets to almost -300 °F (-184 °C). Too hot during the day and too cold at night.
The Moon's Surface
The moon's surface contains no water to speak of. In 1998 a NASA spacecraft, the Lunar Prospector, found frozen water at the Moon's North and South poles. It is speculated that this ice may have come from comets because comets are largely composed of ice and space dust and no other water appears to be present on the moon.
In addition to the craters, the moon has small volcanic domes and lava rilles. It doesn't seem to have volcanic activity now, but apparently it did in the distant past.
The surface of the moon is covered by powdery soil called regolith that resulted from the constant bombardment of the Moon's surface by meteoroids, asteroids, and comets.
Half of the moon is always in sunlight and the other half is in darkness. The Moon also has a lunar near and far side that don't correspond to the light and dark side. The near side of the Moon ALWAYS faces Earth and the far side NEVER faces Earth. This means that we always see the near side when we look at the Moon and never see the far side. The reason we can only see one side of the Moon is because the Moon's rotation period is about the same as Earth's. Because we only see the near side, the way the Moon looks to us depends on whether or not the near side happens to be in lightness and darkness.
The Moon's Phases
The lunar cycle, which lasts about a month, begins with the new Moon. During a new Moon the near side of the Moon faces away from the sun making it the dark side. Because we can only see the near side from Earth, during this phase the Moon is not visible from Earth.
A few days later when a small portion of the near side of the Moon moves into sunlight and becomes visible from Earth we can see a small portion of the Moon. This phase is called the crescent Moon or waxing crescent.
The Moon continues moving through it's orbit and eventually half of the near side of the Moon is visible from Earth. This is called the quarter Moon, despite that to us it looks like a half Moon. The reason astronomers call this a quarter Moon and not a half Moon is because, although we can see half of the near side, there is still another half of the Moon (the far side) that we can't see and so in reality we are only seeing a quarter of the Moon. Yes, it is confusing.
When the near side of the Moon becomes more lit up than a quarter Moon, but less than a full Moon, it is called a waxing gibbous Moon.
Eventually the entire near side is in sunlight and this is, of course, the full Moon.
As the near side then moves out of the sunlight it becomes a waning gibbous (less than a full Moon, but more than a quarter Moon is visible to us). When we once again get to the quarter Moon it is called the last quarter. As the Moon looks like a crescent again it is called the waning crescent, and finally it becomes a new Moon and the lunar cycle begins again.
Photo of full and crescent Moons: Courtesy of NASA Johnson Space Center.
Photo of Moon's Surface: Courtesy of NASA Glenn Research Center.