2.6 Synchronous rotation

Because most moons in the Solar System are in close orbit around much larger planetary bodies, over time the speed of a moon’s rotation is decreased as the tidal, or gravitational, pull between the two bodies drags on the moon’s spin. Eventually, this slows down a moon’s rotation so much that it completes only one rotation about its axis per orbit, resulting in the same side of the moon facing its planet at all times (known as captured rotation, or synchronous rotation). With the Earth and the Moon, these competing tidal forces have also exerted drag on the Earth’s rotation, slowing its rate of spin about its axis and thus lengthening a day by almost two milliseconds per century.

Tidal forces affect you too. While standing on the Earth’s surface, your head is nearly two metres further away from the centre of the Earth than your feet are. The Earth’s gravitational field can be treated as if all the mass were concentrated at the planet’s centre. Since the force of gravity decreases as the distance increases, your feet are pulled down slightly more strongly than your head. The anatomical consequences of this stretching are negligible for you, but for a much more extended body such as a moon, the physical consequences can be quite noticeable and in one case, as you will see later, very severe.

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60-second Adventures in Astronomy Number 5, The Rotating Moon. Whenever you look at the Moon from Earth, it always looks similar. Different parts are illuminated at different times, but oddly, we always see the same features. The Moon never turns its back on us, much like the rules of etiquette when you visit the Queen.

So does this mean the Moon doesn’t rotate? Well, no, because then as it orbited us, we would see first its front, then its left side, and then its rear. What actually happens is that the Moon rotates exactly once every orbit, which takes a bit less than a month. So though you’d see it spinning from an outside perspective, from the Earth we always see the same side. In fact, we didn’t get a proper view of the far side of the Moon until 1959, thanks to the Soviet space probe.

The Moon used to spin a lot faster, but over millions of years the gravitational pull or tidal force from the Earth has slowed the Moon down. The same thing has happened to most moons of large planets. But it doesn’t work both ways, because while the Moon is spinning once every orbit, the Earth is rotating about 30 times faster. So from a vantage point on the Moon, you’d get to see us from all sides if you stuck around long enough.

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Ever wondered where the Universe came from? Or more importantly, where it’s headed? Voiced by David Mitchell, this series of 60 second animations [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] examines different scientific concepts from the big bang to relativity, from black holes to dark matter. The series also explores the possibility of life beyond Earth and considers why David Bowie is still none the wiser about life on Mars.