
Do you ever feel like some days drag on longer than others? That feeling may be psychological, but actual day length really does fluctuate--by a fraction of a millisecond. (A millisecond is one thousandth of a second)
The length of a day, which is measured by the time it takes Earth to rotate once on its axis, can be measured to an accuracy of about 10 microseconds, or 10 millionths of a second. Earth's rotational rate depends on the distribution of mass across its surface.
This change in rotational speed, though it is minimal, has been observed for centuries. In 1695 English astronomer Sir Edmund Halley (who also discovered the eponymous periodic comet) hypothesized that the moon was accelerating in its orbit. In reality, Earth's rotation was slowing down, making it appear that the moon was gathering speed.
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