Scientists have known for a long time that the Earth's rotation has been slowing down. They've used all sorts of different methods, from the time and location of ancient eclipses to super-precise measurements of the length of a day. In fact, the Earth's rotation isn't even steady over the course of a century — it slows down and speeds up ever so slightly, possibly because of glitches in the movement of our planet's molten core. Generally, however, it's accepted that the biggest culprit for this slowdown is our own moon, whose gravity tugs on the Earth's tidal bulge and takes the wind out of its sails like a spinning figure skater stretching her arms out to slow down.

Now, researchers from the University of Wisconsin Madison have used statistical tools and geological analysis to reach far, far back in Earth's history and discover just how much our rotation has slowed over time. See, the moon isn't the only thing exerting force on our planet — other bodies in our solar system do too, and together, these forces create variations in Earth's orbit around the sun, its rotation, and even its wobble on its axis. Those variations are known as Milankovitch cycles, and they determine how the sun's rays strike Earth, and thereby Earth's climate. Variations in the climate leave evidence in the geologic record, which scientists can analyze to see our planet's climate history, and therefore its Milankovitch cycles — at least in the last hundreds of millions of years or so.

But scientists lack the precision to look any further back in time. If you could figure out just how the moon and planets were moving around the Earth billions of years ago, you might be able to learn about these cycles that way. But that's incredibly complicated — which is why it's lucky that a Columbia professor had developed a statistical method that was perfect for this problem. For a study they published in the Proceedings of the National Academy of Sciences last month, University of Wisconsin Madison Geoscience Professor Stephen Meyers and Columbia Lamont Research Professor Alberto Malinverno teamed up to combine Malinverno's new method with astronomical theory, geologic data, and some statistical magic.

They used this approach on two ancient rock layers, one from the 1.4 billion-year-old Xiamaling Formation in Northern China, and another from a 55 million-year-old layer from Walvis Ridge in the southern Atlantic Ocean. That let them determine wobbles in Earth's axis and the shape of its orbit both recently and long, long ago. Sure enough, the method was a success, and they learned just how much the Earth has slowed: 1.4 billion years ago, a day on Earth lasted a little more than 18 hours, and the moon was roughly 43,500 kilometers closer.

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