THE MOON IS EARTH'S only satellite, about 239,000 miles (384,000 km) away. Next to the Sun it is the brightest object in our sky,more than 2,000 times as bright asVenus. Even without a telescope, we can see large areas on the Moon that are darker than the rest. Early observers imagined these might be seas, and they were given names such as the Sea of Tranquillity. We now know that there is neither liquid water nor an atmosphere on the Moon. The so-called" seas" are plains of volcanic rock where molten lava flowed into huge depressions caused by giant meteorites, then solidified.Volcanic activity on the moon ceased about two billion years ago.

The Moon's craters were formed between 3.5 and 4.5 billion years ago by the impact of countless meteorites. These impact craters are all named after famous astronomers and philosophers. Because the Moon has no atmosphere, there has been little erosion of its surface. This plaster model shows Copernicus crater, which is 56 miles (90 km) across and 11,000 ft (3,352 m) deep. Inside the crater there are mountains with peaks 8 miles (5 km) above the crater's floor.

TIDE TABLES: The pull of the Moon's gravity (p.21), and to a lesser extent, the Sun's, causes the water of the seas on Earth to rise and fall.This effect is called a tide. When the Sun, the Moon, and Earth are all aligned at a new or full moon, the tidal "pull"is the greatest. These are called spring tides. When the Sun and the Moon are at right angles to each other, they produce smaller pulls called neap tides. This compendium (1569) contains plates with tables indicating the tides of some European cities. It was an essential instrument for sailors entering harbor.

PHASES OF THE MOON: The phases of the Moon are caused by the constantly changing series of angles formed by the Sun and the Moon as the Moon revolves around Earth. When the Moon and the Sun are on opposite sides of Earth, the Sun shines directly on the Moon's surface, resulting in a full moon. When the area of the lit surface increases, the Moon is said to be waxing; as it decreases, it is said to be waning.

SOLAR ECLIPSE: A solar eclipse happens when the Moon passes directly between Earth and the Sun, casting a shadow on the surface of Earth. From an earthly perspective, it looks as if the Moon has blocked out the light of the Sun. Total eclipses of the Sun are very rare in any given location, occurring roughly once every360 years in the same place. However, several solar eclipses may occur each year.

THE SURFACE OF THE MOON: The features on the far side of the Moon were a mystery until the late 1950s.This view of the terrain was taken by the Apollo 11 lunar module in 1969. One of the primary purposes for exploring the Moon was to bring back samples of rock to study them and to discover their origins. The Moon is made up of similar but not identical material to that found on Earth. There is less iron on the Moon, but the major minerals are silicates as they are on Earth (p.43)-though they are slightly different in composition. This discovery supports the most popular theory of the Moon's origin. A small planet, about the size of Mars, is thought to have crashed into Earth about 4.5 billion years ago. The collision tore debris away from both bodies and the Moon formed from this material.