If you were to travel back in time more than a few hundred years and look up at the night sky, you might find that, although certainly familiar, it wouldn't look quite as it does today. Everything in the universe is in motion, and the constellations seen by the ancient Greeks, Romans, and Chinese have shifted since their stories were first told. One thing, however, would seem quite unchanged: the Moon.
It seems likely that no object in human history has been studied more thoroughly than our only natural satellite. More than three thousand years ago, we learned to predict the timing of lunar eclipses, periodic alignments of the Earth, Moon and Sun. It was all the way back in the second century BCE that Seleucus of Seleucia first suggested that tides on Earth were caused by the Moon. In ninth century Persia, Habash al-Hasib al-Marwazi of Baghdad became the first to correctly compute the size and distance of our companion. In short, we've spent a lot of time learning a lot of things about the Moon. And yet, it's taken all of human history to notice one curious fact: the Moon is shaped like a lemon!
Of course, I'm being a little overdramatic and sensational here. Like all solid bodies above a certain size, the Moon is, to a good approximation, a sphere. The Earth itself is round to about one third of one percent, hardly like your typical lemon. The most common cause of non-roundness in the solar system is rotation. If you spin a sphere, it tends to become oblate, or a bit larger at the equator and shorter at the poles. The Earth spins at the relatively pedestrian rate of once per day, which results in a polar radius that is about 21 km (13 mi) shorter than its equatorial one. Saturn, a squishy gas planet spinning much more quickly, is nearly 10% larger at its equator!
Measuring the oblateness of the Moon is unusually tricky because its surface is disrupted by a series of enormous craters. Some of these craters are similar in size to the Moon itself, which means they severely distort any idea of roundness to begin with. With this in mind, precise measurements have shown that, while about three times more spherical than the Earth, the size of the Moon changes by about 2 km (1.2 mi) from pole to equator. The Moon, however, spins extremely slowly and so this difference has long puzzled astronomers.
In a paper published this week in Nature, a group of researchers suggest a pretty sensible answer: tides. Just as gravitational effects from the Moon lift up the surface of the ocean at high tide, gravity from the Earth also pulls on the Moon. Rock, of course, is a lot harder to budge than water, but the lunar surface wasn't always as rigid as it it today. As the surface formed billions of years ago, it would have been hot and flowing like lava. At that point, the Earth's gravity would indeed have created surface tides on the Moon. This process could have continued for a long time, but eventually the lunar surface cooled. When it did, the equator happened to be frozen at high tide, and it remains that way today.
Tides play an important role all over the solar system. Jupiter, with a mass hundreds of times greater than the Earth, does today what our planet cannot: it violently lifts up the solid surface of its nearest moon, Io. As its surface heaves, friction generates a tremendous amount of excess heat and this heat powers the largest and more prolific active volcanoes in the solar system. Tidal forces from Jupiter and Saturn also act on their moons Europa and Enceladus, helping to sustain liquid water oceans in regions of space more than a hundred degrees colder than Antarctica. So, next time you're at the beach and you see the tide coming in, it's just one small act in a world full of tides!