Earth’s day isn’t exactly 24 hours. The reason may lie deep inside the planet
Most of us think of a day as a fixed 24 hours. In reality, Earth's rotation is not perfectly constant.
The difference is tiny, measured in milliseconds, but physicists can detect it. Over time, some days become slightly shorter or longer depending on subtle changes deep inside the planet.
For roughly three decades, scientists have known that Earth's liquid core does not always rotate at the same rate. Changes in the planet's magnetic field have revealed that the liquid core can gradually speed up over several decades, then slow down during the following decades.
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