The moon is the only natural satellite of Earth. The moon orbits the Earth from 384,400 km and has an average speed of 3700 km per hour. It has a diameter of 3476 km, which is about ¼ that of the Earth and has a mass of 7.35e22 kg. The moon is the second brightest object in the sky after the sun.
The gravitational forces between the Earth and the moon cause some interesting effects; tides are the most obvious. The moon has no atmosphere, but there is evidence by the United States Department of Defense Clementine spacecraft shows that there maybe water ice in some deep craters near the moon's North and South Pole that are permanently shaded. Most of the moon's surface is covered with regolith, which is a mixture of fine dust and rocky debris produced by meteor impact. There are two types of terrain on the moon. One is the heavily cratered and very old highlands. The other is the relatively smooth and younger craters that were flooded with molten lava.
Throughout the 19th and 20th centuries, visual exploration through powerful telescopes has yielded a fairly comprehensive picture of the visible side of the moon. The hitherto unseen far side of the moon was first revealed to the world in October 1959 through photographs made by the Soviet Lunik III spacecraft. These photographs showed that the far side of the moon is similar to the near side except that large lunar maria are absent. Craters are now known to cover the entire moon, ranging in size from huge, ringed maria to those of microscopic size. The entire moon has about 3 trillion craters larger than about 1 m in diameter.
The moon shows different phases as it moves along its orbit around the earth. Half the moon is always in sunlight, just as 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. In the new moon, the face is completely in shadow. About a week later, the moon is in first...
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