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The moon is the earth's natural satellite, orbiting the earth every 29 1\2 days. It keeps the same face permanently turned toward the earth by spinning. With a diameter of 2,170 miles, the moon is about a quarter the size of the earth. It is earth's nearest neighbor at an average distance of 238,080 miles. The dark areas on the moon are called maria, from the Latin mare, meaning sea. At one time they were thought to be filled with water but it is now known that there is no water on the moon, nor has there ever been. The moon is a waterless, airless and lifeless. The surface of the moon heats up to above the boiling point of water in sunlight. During the evenings the temperatures drop to -360 degrees fahrenheit.

One of the most striking features of the moon is its craters. The largest of these craters measures 186 miles in diameter and all are believed to be the result of meteorite impacts. No one is quite sure how the moon was formed. Some believe that it is a bit of the earth that split off while others believe that the moon is a separate body that was captured by the earth's gravity. A third theory says that the earth and moon formed side by side, much as we see them now. Most astronomers support the third theory.
If we could look down from a distant star, we would see the moon orbiting the earth in a path that is slightly out of round. We on earth see the moon go through phases as we see more or less of the orb lighted by the sun. In the crescent phase we see only a sliver that is lighted by the sun. The sun is almost behind the moon at this time and the moon and sun appear close together in the sky. Thus the crescent moon always appears close to sunrise or sunset.
Galileo was the first to discovered the surface features on the moon in 1610, when he turned the newly invented telescope on it. There he discovered smooth, dark areas that look like seas, even though they contain no water. Galileo named lunar seas maria. One half of the moon is always lighted by the sun. But the half that is lighted usually includes part of the moon that we can see and part that we never see. As the month goes on, the angle from the Earth to the moon to the sun changes, so different parts of the moon are lighted. The phase we see depends on how much of the lighted half of the moon is in the half of the moon that we are seeing.
When each monthly cycle begins, only the far side of the moon is lighted. This is when we have a new moon. Many centuries ago, Leonardo da Vinci realized that even the dark part of the moon received some light from the sun that was bounced off the Earth to the moon. The dark part of a crescent moon can sometimes be seen by this "earthshine".
Every 24 hours, as the earth spins on its axis, the moon moves around the Earth in the same direction. The Earth must spin a little longer, usually about 50 minutes, for our view to catch up with the moon. This means that the moon usually rises about 50 minutes later from night to night. Immediately after the new moon the crescent is not visible for a day or so. Knowing the exact time when the moon will become visible depends on the weather. With perfectly weather conditions, the moon is visible even in the daytime. When we begin to see more and more of the lighted half of the moon each night, we say the moon "waxes". During this time the waxing crescent becomes visible in the evening sky near sunset, usually around dusk. It will then set a few hours after the sun. Since the moon has an elliptical orbit around the Earth and since its orbit is tilted, we are sometimes able to see more on one side or another around the moon's edge. These changes in angle, which are known as librations, will allow us to see slightly more than one half of the moon over time.
The first quarter moon is seen a week after the new moon. This is when half the face of the moon we see is lighted by the sun. This phase is also know as a half moon. As the crescent waxes, we can see what is called the Sea of Tranquillity. It is this flat and smooth area that was chosen as the site of the first human landing on the moon by the Apollo II astronauts. Between 1969 and 1972, six American spacecraft, each carrying two astronauts, landed on the moon. These astronauts carried out many experiments on the moon and left behind instruments that would radio results back to earth. Mirrors the astronauts left on the moon are still being used to reflect laser beams back to Earth to help scientist measure the distance between Earth and the moon accurately. They also brought back 843 pounds of rocks which were analyzed by scientist to determine their age. The oldest rocks were 4.42 billion years old which lead scientists to believe that this must have been when the moon's surface solidified.
The line dividing the lighted regions from the shadowed regions on the moon is called the terminator. It is here that shadows are longest causing craters and mountains stand out. Here on earth we calculate the heights of lunar mountains by measuring the heights of their shadows. Explorations have shown that the southern half of the visible side of the moon is covered with highlands. Since the middle of the curved side of the quarter moon is facing the sun, we can calculate the direction toward the sun from looking at the moon .
We see a gibbous moon when the lunar phase is between quarter and full. This moon is thus called because it give us more light than a crescent or half moon. A full moon occurs about two weeks after a new moon and due to the sun's light hitting the center of the full moon straight on, we see little detail in the craters. For amateur astronomers this is a good time to view the lunar seas. The moon is what we call waning when it is past full and the lighted part appears to diminish from night to night. The third quarter moon is phase when the moon rises at midnight. In this phase the moon sets at noon, after it has been visible throughout the morning. After the third quarter moon, the moon rises after midnight and is visible only in the early morning hours before sunrise or in daylight hours. Because of this the waning crescent is seen less often than other phases.
When the moon is full the sun is behind the Earth casting its sunlight over our shoulders directly onto the moon. But because the moon's orbit is tilted, the Earths shadow will usually miss covering the moon. Every few months, the sun, Earth and moon are right in line and we have what is called a lunar eclipse.