When I was a kid my younger sister used to play basketball in a city league and my family would go watch. The game clock would reach the final seconds of each period and the kids in the stands would count down. 3! 2! 1! Zero!
The clocks were such that with less than one minute remaining, the clock would display tenths of a second. This resulted in a problem that bothered me but probably no one else ever. So as soon as (say) the 5 popped up the kids would shout “5!” when in fact there were 5.9 – or really six seconds left. This lead to the inevitable embarrassing moment when everyone would yell “zero!” and a second would elapse as the clock continued to count down from 0.9 before finally buzzing when time expired. Well, I was embarrassed for them anyway.
Today we’re going to have a global instance of a very similar thing. There’s going to be an extra second hanging around at midnight. It happens all at once though, so unless you live where midnight is actually at 00:00:00 UTC the problem won’t interfere with any Times Square style countdowns.
But there is an extra second, and it’s called a leap second. It’s mainly the moon’s fault. Tides rise and fall because the moon’s gravity pulls the water into a tidal bulge which the earth rotates under. This results in friction, which slows down the planet very gradually. Every year the rotation of the earth slows by about 17 microseconds. Over time this lag builds up and eventually the keepers of the various international standard clocks have to add a second in order to keep the clocks in synchronization with the spinning planet. It’s not a smooth slowing however, and so the corrections come irregularly. This year there’s one which will occur just after 5:59:50 p.m. Central Time today. The clocks will go (assuming your computer is clever enough to show it) …5:59:58, 5:59:59, 5:59:60, 6:00:00… with that funky 5:59:60 being the leap second.
The US Naval Observatory. Current home of Dick Cheney, future home of Joe Biden, and permanent home of the US Naval Observatory Master Clock.
As with the whole Pluto business, there’s some controversy about whether these leap seconds are a good idea. Because of the whole irregularity of the earth’s slowing, they can’t be predicted far in advance. That means embedded systems without easy access to internet time servers can’t account for the ones that haven’t happened yet. This can cause problems. On the other hand, there’s nothing requiring those systems to use UTC time. They could use GPS time, Unix time, or any of various other true delta-t standards. Conversion could happen later for non-critical user interface requirements. Me, I’d vote for keeping the leap seconds. I’d hate to eventually saddle our many-times great grandchildren with a clock that read midnight at sunrise.
Bonus Fermi problem! The earth has kinetic energy by virtue of its rotation, so if it’s slowing it’s losing energy at some average rate. What is this rate, in watts? Order-of-magnitude estimates are fine.