The moon is a really neat object. It is the brightest object in the night sky and yet it is one of the least shiny. The surface of the moon is almost black with the face is the darkest. It seems to be about the same size of the sun, yet it is so close to us that it creates most of the tides on the earth. The face on the moon seems to always face the earth, which helped early scientist believe that the earth was the center of everything.
The moon was created early in the history of the solar system. It formed when a large Mars sized planet impacted the earth about four and a half billion years ago. Both the earth and this object had already under gone differation of materials, meaning the heavy elements had sunk to the core and the light had risen to the surface. Both planets were mostly molten too, so when the impact occurred, the center of both masses joined together in the Earth's core, while the lighter material sloshed around with a fair amount of momentum. The momentum allowed a large amount of the body to achieve escape velocity, but not enough to break free entirely. This meant it slowly cooled as the earth cooled, now the lighter the satellite of the heavier.
At first the moon was still quite close and it traveled around the earth once every few days but as it cooled and other impacts occured over both planets surface irregularities developed and one side of the moon became heavier than the other. This side became locked slowly to the earth, and over billions of years the locked side slowed its rotation. As the rotation slowed, it's orbital velocity sped up and the distance between it and the earth spread apart.
The affect of the tidal lock was not one sided, the slowing rotation slowed the rotation of the earth too. The earth spun faster when it formed, but the drag of the Moon's gravity slowed the rotation until we got our 24 hour day. It did not stop there; the moon will continue to slow the earths rotation down until we are both locked facing each other, but that will not be for billions of more years.
Many know that the moon is responsible for the tides in the sea, but they do not know how it does this. The gravitational center of the earth is not at it's center, but is actually between the center of the earth and the center of the moon. It is still in the center of the Earth, but it is off center and since the earth and the moon move in relation to each other the gravitational center also moves. Between the Moon and the Earth, the gravity moves the oceans up towards The Moon, and on the reverse side the reduced gravity allows the oceans to rise as well. There are a couple other types of tides, they are caused by the interaction of a third gravitational force, the sun. When the moon is full or new, the Sun, the Moon and the Earth form a straight line and the tides are bigger, a Spring Tide. When the Moon is half full, the Sun, the Earth and the Moon form a right angle and e tides are reduced, a Neap Tide.
One of the cooler aspects of the moon and the earth is its seasons. There is a 23.5 degree tilt to the Earth's spin which creates the seasons. When the sun appears high in the sky, at night the moon will appear low in the sky and during the winter when the sun is low in the sky, the moon will appear high. Of course this all depends how far away from the equator the observer is. At the equator the sun and moon appear to move very little, but at the poles the moon will rise and set depending on the phases. Rise for the full moon for winter and rise for the new moon in the summer.
And the moon changes size too . . . no it does not!! The moon does fluctuate in its orbit around the earth, sometimes it is closer and sometimes it is further away, but he difference is negligible when compared to it average orbital distance. But it does appear to change size and this size variation is actually easy to explain. Take an airplane as it lands we see it from a thousand meters away, but in one instance it is overhead and the other instance it is near the horizon next to some high rises. Overhead it looks big, but it is difficult in our minds to give it a scale, it could be to our minds be a thousand meters or ten thousand and we would not really know the difference there is nothing near it to make us realize the scale, but when the plane is near a high rise, we see that it is large because we have a reference to judge its size by. The same thing can be said about the moon; close to the ground beside things we know it appears huge but up in the sky where there are no scale objects it looks smaller. If you want more proof, hold your hand out and cover the moon with your thumb, do it at the horizon and when the moon is at its zenith; you will cover the same amount.
The best thing about the moon is that it is there. As Douglas Adams told it in his book, The Hitchhikers Guide to the Galaxy, a planet without a moon or stars would never pine to leave the surface; they would never think of it because their poets, song smiths and writers would never think of it.
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