Although the moon may seem small and insignificant compared to the rest of the universe, its size and location make it very important to the Earth. Other than the sun it is the most visible body in the sky. Because of this, humans have been fascinated by the moon since ancient times. It has been studied both from the Earth and from space. In fact, more than 70 spacecraft have been sent on missions to the moon. In addition, the moon is the only place outside of the Earth that has been visited by humans.
The average distance of the moon from the Earth is about 238,900 miles (384,400 kilometers). This may seem like a great distance. However, compared to the distance of the Earth from other planets, the moon is actually quite close.
The moon is slightly more than one fourth the size of the Earth. It is made of solid rock. The surface is covered with a layer of dust consisting of fine-grained rock fragments. The moon's landscape features include craters, mountain peaks, deep narrow valleys, and plains, which are sometimes called maria.
When looking at the moon from the Earth, some areas appear light-colored and other areas appear dark-colored. The light areas of the moon are the highlands. These highlands are covered with thousands of craters, some of them overlapping one another. Astronomers are not sure about their origin, though many believe that they are the result of meteorites hitting the moon's surface. The highlands also contain mountain ranges. These mountains were given such names as the Alps, Pyrenees, and Carpathians, after mountain ranges on Earth.
The dark areas on the moon are large plains, or maria. The word maria is the plural of the Latin word mare, which means “sea.” It is believed that these plains are craters that were filled with lava billions of years ago. The plains on the moon have names such as Mare Imbrium (Sea of Showers) and Mare Nectaris (Sea of Nectar). Mare Imbrium is the largest plain. It is about 700 miles (1,120 kilometers) in diameter.
The moon's temperature varies greatly depending on whether it is facing the sun. When the surface faces the sun during the moon's day, the average temperature is about 225° F (107° C). At night on the moon, when the sun's rays do not reach the moon's surface, the temperature cools to about –243° F (–153° C).
Like the planets, the moon has two types of movement, known as orbit and spin. The orbit is the path that the moon travels around the Earth. It takes the moon about 27 days to make one orbit of the Earth.
The moon also spins on its axis. The axis is an imaginary line that passes through its poles. It takes the moon about the same amount of time to make one complete revolution on its axis as it takes it to orbit the Earth. For this reason, the same side of the moon always faces the Earth.
When viewed from the Earth, the moon appears to go through different phases. Sometimes it looks like a full circle while other times it appears as only a thin crescent. This is because the moon reflects the light from the sun. As the moon travels around the Earth in its orbit different parts of the moon are exposed to the sun's light. When the moon is on the other side of the Earth from the sun, the sun is shining directly on the moon, and the moon looks full. However, when the moon is between the sun and the Earth, the moon looks dark since the sun is behind the moon. In each cycle of the moon's orbit around the Earth, the moon displays four main phases: new (when the side of the moon that faces the Earth is dark), first quarter, full, and last quarter. It takes the moon about 29 days to complete this cycle.
Many astronomers think that a large body, perhaps the size of the planet Mars, struck the Earth early in the history of the solar system. As a result, a cloud of fragments was thrown out around the Earth. These fragments later combined to form the moon.
Because the moon is the brightest object in the Earth's night sky, it has been observed by people since ancient times. In the 17th century the invention of the telescope allowed the Italian scientist Galileo and others to study the moon more closely. In the same century Isaac Newton discovered a force called gravity. He also figured out that the pull of gravity from the moon affects the level of the seas, causing tides to occur. For the following 300 years scientists continued to study the moon from the Earth.
In 1957, the space age began when the Soviet Union launched an artificial satellite called Sputnik. This gave scientists greater freedom to study bodies in outer space. Throughout the early 1960s, the United States and the Soviet Union launched many spacecraft that flew by, crashed into, or orbited the moon. These missions provided close-up photographs of the moon. In 1966 the Soviet craft Luna 9 became the first spacecraft to land on the moon. This important mission provided photographs of the lunar soil.
From 1966 to 1968, the United States and the Soviet Union continued sending unmanned craft to orbit or land on the moon. These missions paved the way for the historic landing of Apollo 11 on the moon.
On July 20, 1969, U.S. astronauts Neil Armstrong and Edwin “Buzz” Aldrin, Jr., of the Apollo 11 mission, were the first men to walk on the moon. There they set up experiments and took photographs. Armstrong and Aldrin returned to Earth with about 48 pounds (22 kilograms) of rock and soil samples that they had collected.
From 1969 to 1972, the United States landed five more crews on the moon. The Soviet Union placed vehicles operated by robots on the lunar surface until 1976. From 1977 until 1990, no missions to the moon occurred as the United States and the Soviet Union studied other parts of the solar system. In 1990, the Japanese Space Agency launched a satellite that was put into orbit around the moon.
The next major U.S. mission was the Lunar Prospector, which was launched in 1998. It was a small, unmanned spacecraft that orbited the moon and mapped its surface. After a year, the craft was deliberately crashed into the moon to look for evidence of water. It did not find any water.
