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4.1.6 Looking at the Moon

As mentioned in Week 1, we observe the same side of the Moon at all times due to synchronous rotation in which the Moon takes almost the same amount of time to complete one rotation on its axis as it does to complete one orbit around the Earth. If the orbit of the Moon was completely circular around the Earth, we would only see exactly half of the Moon. In fact, the orbit is not completely circular: it is elliptical. This causes the Moon to be closer to the Earth at some points in its orbit than at other points. Because the orbit is elliptical, the Moon’s orbital speed is not the same all the way round. The result of the changing speed is that the Moon appears to wobble slightly during its orbit, a process known as libration. This libration allows us to see about 60% of the Moon’s surface, as demonstrated in the animation.

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