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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.

The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth's surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher.

The Moon is the only place in our solar system, other than Earth, where humans have visited. The Moon is like a desert with plains, mountains, and valleys. It also has many craters, which are holes created when space objects hit the Moon's surface at a high speed. There is no air to breathe on the Moon. Recently water ice was discovered at the poles (or top and bottom) of the Moon. The ice is buried beneath some of the dust of the Moon's surface. Scientists think the ice may be left over from a comet that once collided with the Moon.
 

The Moon travels around the Earth in an oval shaped orbit. Scientists think the Moon was formed long ago when Earth collided with another space object. The collision may have caused a big chunk of rocky material to be thrown out into space to form the Moon.

The Moon is a little lopsided. Its crust is thicker on one side than the other. The Moon is much smaller than the Earth. However, the pull of its gravity can still affect the Earth's ocean tides.
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