FIGURE A-1. The Earth and the Moon orbit their mutual center of mass, which is termed the barycenter (B). The barycenter happens to be within the Earth because our planet is so much bigger than the Moon. The two bodies are not here shown to scale compared with their separation.

the equator you are whizzing along at more than a thousand miles an hour, as you spin around the Earth's center. In our everyday frames of reference we are unaware of such movements. Wobble and change speed we certainly do, as we revolve around the Sun.

The Earth and the Moon rotate about the barycenter quite independently of the fact that they both spin on their central axes—the Earth once a day and the Moon, it happens, exactly once a month. The Moon therefore keeps basically the same face towards us at all times. We say that it is "tidally locked." Over the eons the lunar spin rate has been damped by Earth's gravity, because the Moon's mass distribution is not uniform. There is a greater density beneath the lunar nearside, displacing its center of mass away from its axis of symmetry, and the pull of the Earth keeps that greater mass directed towards us.

Finally, I wrote above that the barycenter is about 2,900 miles from the Earth's center, but actually its position varies. This is because the separation of the Earth and Moon changes, the lunar orbit being noncircular. We will learn more about this below.

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