1. Chapter Four The Moon: The Earth’s Closest Neighbor

2. The Moon’s Formation The Moon is approximately 4.5 billion years old – this is just as old as the Earth, which suggests that they were formed at about the same time. The origin of the Moon is still being debated. – One theory proposes that the Moon formed elsewhere in the solar system and was captured by the Earth’s gravity as it passed close to our planet. – Another theory suggests that debris orbiting the Earth that was left over from the formation of the planet eventually came together to form the Moon. – The latest theory of how the Moon formed is gaining the most popularity  this theory states that a large object, perhaps as large as the planet Mars, struck the Earth and sent a large amount of the Earth’s crust into space.

3. Evidence to support this theory comes from rock samples recovered from the Moon. – They reveal that the Moon is mainly composed of silicate minerals and is deficient in iron. Definition: a chemical compound that is composed of atoms of oxygen and silicon Definition: lacking something essential to life – The lack of iron on the Moon, as compared with Earth, suggests that it is formed from the outer crust of the Earth, which contains much less iron than is located in its core. The Composition of the Moon The rocks that were brought back from the Moon are very similar to volcanic rocks produced on the Earth. Definition: igneous rocks that form from cooled lava produced by a volcano

4. The volcanic eruptions that formed these rocks on the Moon occurred early in the Moon’s history, between 4 and 2.5 billion years ago. –These ancient eruptions created large plains of basalt lava that are called mares. Definition: a common fine-grained volcanic rock that is dark, mafic and dense Definition: a Latin word for “seas” used to describe the flat dark plains that cover the surface of the Moon –Since the formation of the mares, the Moon has had no known volcanic eruptions. –The most recognizable feature of the Moon’s surface are its impact craters. Definition: large bowl-like depressions that are left on the surface of a celestial object as the result of an impact by another celestial object, usually an asteroid or comet

5. The Moon’s Surface Astronauts who landed on the Moon discovered that the surface is covered with a fine volcanic dust, which is formed by the impact craters. The Moon’s surface has no atmosphere, and its gravity is 17 percent of the Earth’s gravity. –The average surface temperature of the moon varies greatly during the day. –At noon the temperature on the Moon can reach almost 200oF (93 o C), and during the lunar night the temperature plummets to lower than -250 o F (-157 o C). Until recently it was believed that the Moon had no water, but explorations of the Moon’s polar regions suggest that there might be some frozen water on the Moon.

6. The Moon’s diameter is approximately 2,160 miles, which is roughly 27 percent of the Earth’s diameter. The Moon is approximately 238,866 miles from Earth and completes one orbit around the Earth in 27 days and 7 hours. –The period of the Moon’s revolution around the Earth and the Moon’s rotation on its own axis are the same; therefore the same side of the Moon always faces the Earth. The Moon’s Orbit As the Moon orbits the Earth, one side of it is always illuminated by the Sun. –This causes the different Moon phases as viewed from the Earth’s surface. –Different phases of the Moon occur at different points in its revolution around the Earth.