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Planets and moons


The MoonThe Moon The Moon, our nearest neighbour in space, is neither a star nor a planet. It is a ball of rock that travels around Earth, taking about 27 days to complete the circle. The same side of the Earth's Moon always faces the Earth, because the Moon is "tidally locked" to Earth: Earth's gravitational pull holds it in place. The Moon takes the same time to spin on its own axis as it does to orbit Earth. In turn, its gravitational pull, along with that of the Sun, causes tides on Earth. The Moon is the brightest object in the night sky as seen from Earth, although the light it “shines” is actually reflected from the Sun. The Moon is so far the only other world in the Solar System that human beings have set foot on.

Formation of the Moon

The sequence of events in the formation of the MoonThe sequence of events in the formation of the MoonThis diagram shows the sequence of events in the formation of both the Earth and Moon. Soon after the Earth had formed, 4.5 billion years ago, a planetesimal (1) the size of Mars, called Theia, collided with it and exploded (2). The impact “splashed” vast amounts of debris into space, surrounding the Earth. The fragments collected in orbit round the Earth, which was by now a ball of melted rock (3). The Earth’s surface later cooled and turned back to solid rock (4). The orbiting fragments compacted together to form the Moon (5, 6).



The Moon is the fifth largest moon in the Solar System.

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