The Moon

One of our fairy tales tells of a man who gathered firewood on a Sunday, even though people were not allowed to work on Sundays. As a result, he was banished to the moon and had to stay there forever. And if you looked up at the moon on clear nights long ago, you believed you could actually see him.

The Moon has always fascinated us. People have written poems and sung songs about it. Some believed that more babies were born during a full moon, or that plants should only be planted or harvested during certain phases of the Moon. No one knew what its far side looked like. The Moon was surrounded by mysteries and speculation.

Today we know that we can see the moon only because it reflects the sun’s light. One half is illuminated by the sun, while the other half is not. But we can only ever see part of the moon’s sunlit side. That’s because the moon is sometimes closer to the sun than Earth is, and sometimes farther away. So once a month it appears to us as a new moon, after which the crescent grows a little wider each day. Eventually it becomes a full moon, then wanes until it is barely visible as a new moon again.

That’s because the Moon orbits the Earth. It takes 29.5 days to complete one orbit. During that same time, it rotates once on its axis. As a result, we always see the same side of the Moon. The Moon is much smaller than the Earth and is an average of 384,000 kilometers from it.

The Moon formed about 4.5 billion years ago. At the time, a large celestial body called Theia is thought to have struck Earth from the side. Huge chunks were torn from the young, extremely hot Earth, which was only 100 million years old at the time.

Together with the remains of Theia, they orbited Earth. Gravity brought these pieces together again, and eventually they formed the Moon.

Earth is one of the eight planets in our solar system. Mercury and Venus have no moons, while Mars is orbited by two small moons. Jupiter and Saturn each have around 80 moons, and the distant planets Uranus and Neptune are also accompanied by numerous moons.

The moon is covered in craters. Some were formed by volcanoes, but most were caused by meteorites or comets crashing into the surface. Because the moon has no atmosphere and no water flows on its surface, its surface never changes. Some of the craters are therefore millions of years old.

The Moon’s craters are circular, usually shallow, and surrounded by raised rims. The largest is the South Pole–Aitken Basin. It is 2,240 kilometres across and up to 13 kilometres deep. It covers a large area of the Moon, which itself is only 3,474 kilometres in diameter.

On the side of the Moon facing the Sun, temperatures can reach 130°C, while on the side facing away from the Sun, they can drop to -160°C. These extreme differences are due to the Moon having no protective atmosphere to regulate temperatures.

Water has been discovered at the Moon’s North and South Poles, but only in the form of ice.

Without the Moon, life on Earth probably would not be possible—or at least it would have developed very differently. The Earth and Moon attract each other. These forces slow the rate at which Earth spins on its axis. Without the Moon, an Earth day would last only about six hours, and Earth’s axis would wobble every few million years.

Without the light of the Moon, our nights would be pitch-black. It reflects sunlight, making the nights bright when the Moon is out. Its gravitational pull helps create the tides along our coasts.

The Moon is the only celestial body besides Earth that humans have ever set foot on. On July 21, 1969, two people landed on the Moon for the first time. In the United States, it was still July 20. Ten more astronauts followed between 1969 and 1972. They brought back 382 kg of moon rocks.