- Earth has been falling behind the atomic time at a rate of about 2 milliseconds per day.
- This Saturday will mark the 25th time a leap second has been added since the practice started in 1972.
This Saturday, June 30, expect a lengthier day, as an extra leap second will be added to Earthlings' clocks.
What's behind this leap second? The ever-so-slight slowing of Earth's rotation, or the 24-hour spin that brings the sun into our skies every morning.
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Historically, humans based time on the average rotation of the Earth relative to other celestial bodies, with the second defined by this frame of reference. However, the invention of atomic clocks - accurate to about one second in 200 million years - brought about a definition of a second independent of Earth's rotation. Instead, they're based on a consistent signal emitted by electrons changing energy states within an atom.
Earth has been falling behind the atomic time at a rate of about 2 milliseconds per day, currently trailing the atomic time by six-tenths of a second. Every now and then a leap second must be added to atomic clocks (and thus all of our clocks) to keep in sync with Earth's oddball rotation.