Formation of the Moon
Around 4.5 billion years ago, it is believed that a planet named Theia (after the mother of the goddess of the Moon in Greek Mythology) smashed straight into Earth, being obliterated on impact due to its smaller size. In this collision, lighter elements like oxygen and silicon were instantly vaporised by the heat and ejected into space, whilst heavier elements such as lead and iron carried on moving, finally stopping at the centre of the Earth. The heavier elements became the Earth's core and the lighter elements gradually coalesced due to gravity as they cooled to a molten state, eventually cooling down to a solid ball orbiting the Earth, later named the “Moon” by mankind.
Looking up now, it's hard to imagine an event such as the 'Big Splash' occurring and, at first, it's difficult to see how we could confirm such a hypothesis. Part of the supporting evidence came in the form of rocks, plucked from the Moon's surface by NASA astronauts on the Apollo missions. In these rocks, the ratios of the different isotopes of oxygen were similar to the ratios of rocks on Earth, indicating that matter from the early Earth must have been ejected into space. Other pieces of supporting evidence include the previous molten surface of the Moon and similarities between the orientations of the Earth's rotation and the Moon's orbit.
The Moon is covered by several different features which it has obtained during its lifetime. Whilst it does have many specific features of interest (see right) it also has lots of interesting general features. The main general features are the smooth, darker seas (maria in Latin), formed by volcanic activity and the rocky, lighter highlands (terrae in Latin), formed longer ago than the seas so more heavily cratered. Less well known general features include rilles (small lines where lava previously ran) and wrinkle ridges (raised lines along the surface caused by tectonic activity).
Because the Moon's rotation period and orbital period are roughly the same, we always see the same side of the Moon. The shape of the visible part of this side (phase) depends on where the Moon is in its orbit. These are the 8 main phases of the Moon:
Important Facts and Figures about the Moon