Hate leap seconds? Imagine a negative one
You’d think that I’d be done writing about time, leap seconds and atomic clocks. Heck, *I* thought I was about done writing about those topics already too. But, alas, it was not meant to be. Earth has found a way to drag me back into this fascinating mess.
So here’s what brought me back to this mess — a simple line chart published by the International Earth Rotation and Reference Systems (IERS) organization:
This plot is the difference between UT1 and UTC in seconds. And to make sure we’re all on the same page in time scale terminology, a quick review:
UT1, a.k.a. “Universal Time”, is the modern equivalent of “solar time” and is determined when a specific set of distant quasars pass specific points in the sky. Solar time used to be measured when the sun was directly overhead on the Prime Meridian, but giant blinding fuzzy balls of gas in the sky is harder to measure precisely than extremely distant quasars that essentially act as point sources. It’s important to know that there are exactly 86400 (24*60*60) UT1 seconds in a UT1 day. If the rotation of the Earth speeds up/slows down, the definition of a UT1 second changes since the number of seconds in a day is constant.
UTC, a.k.a. Coordinated Universal Time, is an atomic standard of time. It’s based off of TAI (International Atomic Time), which is atomic seconds (so the SI definition of 9,192,631,770 transitions of hyperfine states of cesium atoms from ground state). There are also 86400 atomic seconds in a TAI day. Here, atomic seconds are fixed by definition, as well as the day.
