Translations of Encyclopedia about Geology

Moon, the earth's companion

The moon is our constant companion on our journey through the universe. Its diameter is about one-fourth of the earth's diameter. The earth and the moon are sometimes described as a "dual planet". Due to its great mass, it causes the formation of the tide. Its mass, however, is not great enough to retain atmosphere. On the day-side, the temperature on the moon reaches up to 100 degrees Celsius, while on the night-side it falls to –170 degrees Celsius.

The diameter of the moon measures 3476 kilometres. Its average distance from the earth is 384.405 kilometres. Its orbit is not a perfect circle, so its distance from the earth fluctuates. This means that its speed also fluctuates. When the moon gets closer to the earth, it is faster, when it is further away, it is slower. Thus it happens, that the rotation of the moon sometimes lags behind the orbit and at that time we can see a small portion of its other side.

We do not have any detailed knowledge concerning the moon's interior. We do know, that its solid crust consists of a rocky material similar to granite. Its mantle, underneath the crust, also consists of rock. Its mantle surrounds a partially-molten layer, which sometimes produces "moonquakes". The core of the moon is probably metallic. It may be composed of several kilometres of heavy iron mass, which would explain the existence of several magnetic regions on the moon. However, we have not yet been able to prove it.

The far side of the moon, which is turned away from the earth, is full of craters. This indicates numerous collisions with meteorites. On the near side of the moon, the side that faces the earth, we can see deep depressions, or "mare." This word means "sea," because earlier researchers thought that these dark spots of the moon were bodies of water. In addition to these basins, there are lunar continents, heavily scarred by craters.

The round "mare" and the craters indicate, that there have been collisions with other heavenly bodies. These collisions were sometimes so violent, that they penetrated the lunar crust and liberated masses of lava, which spilled on the lunar surface.

The moon completes one full rotation on its axis every 27,3 days. It is the same length of time that the moon requires for one orbit around the earth, which means that the same side of the moon faces the earth all the time. Still, every time we see the moon a little different. The reason is, that every night the moon is illuminated by the sun in a slightly different way and every time it reflects earth's light differently. This creates different phases. When there is "new moon," the moon is invisible, because the side facing the earth does not receive any sunlight. Its night-side is turned towards the earth. The next 28 days the moon waxes and wanes. Full moon occurs when the earth is exactly between the sun and the moon. Then the side that is facing the earth is completely illuminated

However, sometimes the moon enters the earth's shadow and is invisible to our eye. This is called lunar eclipse.

There are several theories concerning the origins of the moon. According to the fission theory, one piece of the earth was hurled into the space and became the moon. The second theory is that the earth captured the moon, which was previously an independent planet, while the third one proposes that the moon and the earth formed simultaneously.

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