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The moon

http://veimages.gsfc.nasa.gov/9297/a000676_pre.jpg

            The Moon (Luna in Latin) is the only one natural satellite of Earth, reaching a distance of approximately 384,405 km of our planet, just over 1-second light.
The Moon has no atmosphere, and has some water in the solid state (in the form of ice crystals). Since there is no air, there is no erosion and the surface of the Moon is still intact for millions of years. It is only affected by collisions with meteorites.
            The Moon is primarily responsible for the effects of tides that occur on Earth. You could say that this effect is the tendency of ocean follow the orbital motion of the Moon, where this causes a friction with the bottom of the oceans, slowing the rotation of Earth around 0.002 s per century, and as a consequence, removing the Moon from our planet on average 3 cm per year.

http://i93.photobucket.com/albums/l76/tt_nyx/imagens/lua.jpg

            Proportionally is the largest natural satellite in our Solar System. Its mass is so significant in relation to the mass of Earth that the axis of rotation of the Earth-Moon system is far from the central axis of rotation of the Earth. Some astronomers use this argument to say that we live in one of the components of a double planet, because to be a double planetary system, its axis of rotation is required to be out of the two bodies. In any event, the presence of the Moon acts stabilizing the rotation of the Earth.
Data from the Moon:

• The Moon has radius of 1,738 kilometers, approximately 27% of the Earth radius.
• Area of 7, 4% of Earth's area.
• Volume of 2.0% of Earth's volume and mass of 1.23% of Earth's mass.
• We can see 500000 craters on the Moon looking up from the Earth.

The Wonderful World of Astronomy - ThinkQuest
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2009