About 4.5 billion years ago, an asteroid the size of the planet Mars hit the Earth in formation, tearing off some of the mantle.
Most of the ejected material first forms a dense ring around the Earth, which eventually coalesces to form a single body: the Moon.
The Moon is in orbit around the Earth from which it is gradually moving away. It is also rotating on itself and its rotation has slowed to synchronize with its earthly revolution, which means that it always shows us the same side: its visible face.
The Moon suffered a strong meteoritic bombardment during its first 750 million years, then intense volcanism, of which the rise of magma created vast dark plains, called seas (mare i>), because it was initially believed that they were bodies of water: Sea of Tranquility, Sea of Serenity, Sea of Rains,...
One of the most recent major impact craters is the so-called Tycho, which is about 100 million years old: it is observable in the lower part of the visible face of the Moon.
