The Earth's satellite: The Moon

The Moon: Earth's Natural Satellite

The Moon is astronomically very close to the Earth. The average distance between the Earth and the Sun is about 150 million kilometers (93 million miles), while the average distance between the Earth and the Moon is only about 384,400 kilometers (238,900 miles).

Smaller celestial bodies that orbit larger ones are called satellites. The Moon is Earth's only natural satellite. Its diameter is about one-quarter that of the Earth, and its mass is about one-eightieth of Earth's. Its surface is covered with mountains, valleys, and countless impact craters formed by meteorite collisions over billions of years. It also contains vast plains of solidified lava created by ancient volcanic activity.

The Moon has almost no atmosphere and no liquid water. As a result, sound cannot travel there, and the sky appears black even during the daytime. Temperatures on the sunlit side can rise above 100°C (212°F), while shaded or nighttime areas can drop to around −170°C (−274°F). Astronauts who visited the Moon relied on specially designed spacesuits and life-support systems to survive these extreme conditions.

Although the Moon appears motionless in the night sky, it is constantly moving. It orbits the Earth at an average speed of about 3,700 kilometers (2,300 miles) per hour while traveling around the Sun together with the Earth. The Moon rotates once on its axis in about 27.3 days, the same amount of time it takes to complete one orbit around the Earth. Because these two periods are nearly identical, the Moon always shows the same face to the Earth. This phenomenon is known as tidal locking.

Long before humans landed on the Moon, astronomers had mapped the side visible from Earth using powerful telescopes. The far side of the Moon was mapped later with the help of spacecraft sent into lunar orbit.

The Phases of the Moon

The Moon does not produce its own light. Instead, it reflects sunlight. Like the Earth, only half of the Moon is illuminated by the Sun at any given time. As the Moon orbits the Earth, the amount of its illuminated surface that we can see changes. This is why the Moon appears as a full circle, a half moon, a crescent, or sometimes seems to disappear completely.

The four main phases of the Moon are:

• New Moon
• First Quarter
• Full Moon
• Last Quarter

The time between one New Moon and the next is approximately 29.5 days.

Why Do We Weigh Less on the Moon?

Because the Moon is much smaller than the Earth, its gravitational pull is much weaker. The Moon's gravity is only about one-sixth as strong as Earth's. This means that a person who weighs 60 kilograms on Earth would weigh the equivalent of about 10 kilograms on the Moon. As a result, astronauts can jump much higher and move heavy objects much more easily than they can on Earth.