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A long time ago, people thought that Earth stood still while the Sun traveled around it each day. Today, we know that Earth moves around the Sun. It also spins, or rotates. Earth’s rotation causes day and night. One complete rotation takes 24 hours, or one whole day.
Earth rotates about its axis. An axis is a real or imaginary line that a spinning object turns around. Earth’s axis is an imaginary line that connects the North Pole and South Pole. It is tilted at an angle of 23.5 degrees. Some scientists think that long ago, an object from space hit Earth and pushed it on its side.
Earth not only rotates, it also revolves around the Sun. To revolve is to move in a circular or near circular path around something else. This path is called an orbit. Earth’s orbit is shaped like an ellipse, which is a flattened circle. Earth takes a little over 365 days, or one year, to complete its orbit around the Sun.
As Earth revolves around the Sun, its axis is always tilted in the same direction. This means that different parts of Earth get different amounts of sunlight throughout the year, causing seasons. In June, the North Pole is tilted toward the Sun. The Sun’s rays strike Earth’s northern half at steep angles. It is summer there. In December, the North pole is tilted away from the Sun. The Sun’s rays strike Earth’s northern half at low angles. It is winter there.
The Moon looks big and bright at night because it is Earth’s closest neighbor in space. The Moon is about ¼ as wide as Earth, and it has 1/6 Earth’s gravity. The Moon does not make its own light. Moonlight is really light the Moon reflects from the Sun.
The Moon revolves around Earth. It takes the Moon about 28 days to complete its orbit. During this time, the Moon seems to have different shapes, called phases. The Moon’s phases include the new Moon, first quarter Moon, full Moon, and last quarter Moon. In fact, one half of the Moon is always lit by the Sun, while the other half is always dark. On Earth, we see different parts of the lit half of the Moon because of the changing positions of Earth, the Moon, and the Sun.
The Moon is dusty and lifeless. It has mountains, flat plains, and many craters. A crater is a hollow area, or pit, in the ground. Chunks of rock and metal from space made most of the craters.
In the 1960s and 1970s, the United States sent astronauts to the Moon. They collected data and samples. Some unmanned missions continue today.
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