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leap year
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. 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 ...
