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[music] Narrator: To most people, there are exactly 24 hours in a day, the ground only

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moves during an earthquake, and the Earth rotates just like it does on a globe.

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And generally speaking, that makes sense! But scientists, who like to find out

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exactly what's going on, know the ground actually moves around quite a bit,

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days are never quite 24 hours, and the Earth actually wobbles on its axis

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in a very particular way as it revolves around the sun. Scientists know

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all this by using a technique called Very Long Baseline Interferometry,

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which is basically a fancy term for using radio dishes to very precisely measure

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the Earth's orientation. VLBI was originally developed

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back in the 60s to take pictures of quasars. Early on, though,

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someone realized that because quasars never really move, you could use them as reference

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points, throw the whole process in reverse, and figure out how all the telescopes

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were moving relative to one another. Basically, when a quasar emits a radio

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wave, that wave reaches different telescopes at different times. For

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astronomy, you'd use a computer to imitate a giant telescope and get a good picture

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of the quasar, but if you instead pay close attention to the time differences,

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you can use geometry to figure out how far apart the telescopes are. And

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by making lots of those measurements, you can start to see how the ground beneath the telescopes

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moves around, when you have to adjust your clock, and that the Earth wobbles on its axis

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as it moseys around the sun. So, the next time you feel like you've had a long

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day, or that your house is a few millimeters from where you last left it, you can

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switch on a bunch of radio telescopes, point them at quasars, and find out just how

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right you are. [music]