Time Stands Still
Tonight’s “leap second” and why the Earth sucks at keeping time
If you stand very still tonight, holding your breath in the still of midnight darkness, you’ll hear the sound of all the clocks in the world pausing for one second. Actually, you probably won’t hear anything, but you should know that today will be one second longer than a normal day. Why?
When trains began to make long-distance travel possible, with schedules dependable down to the minute, there was a worldwide demand for standardized time. So we got Greenwich Mean Time, which defined the measure of a day as the average time of a single rotation of the Earth from the perspective of one Englishman staring up at the sky in Greenwich. In 1820, this just so happened to be 86,400 seconds, or 24 hours.
The problem is that the Earth’s rotation is slowing down, and a “solar day” isn’t exactly 86,400 seconds anymore. The Earth doesn’t care about our time system one bit, apparently.
How does that work? The Moon pulls on the Earth due to its own gravity. When that’s combined with the natural gravity of the Earth, we get two “high-tide” bulges on opposite sides of our planet. But the bulges don’t line up perfectly with the equator, and the Moon actually pulls on the ocean enough to create a tiny amount of friction. That friction is slowing our rotation by about 0.002 seconds per day per century. Eventually the Earth and Moon will be “tidally locked” and each will have a constant face to the other (like the Moon does to Earth today). Phil Plait explains this all pretty well here. Moreover, earthquakes and all sorts of other stuff mean that this “slowing” business is also irregular.
Earth sucks as a timepiece.
Since the 1970’s, our “official time” has been kept by atomic clocks, accurate to one second every 250 million years. We actually changed the official definition of a second to be based on atoms instead of 1/86,400th of a day. But many traditional clocks, not to mention our bodies, are basing their day on day/night averages, and the atomic clocks are basing it on cesium atoms (far more accurately). The day/night clocks are lagging behind! So on a regular basis, the Time Lords of Earth let the atomic clock time pause for one second to bring them closer to sync. That’s a leap second.
If we didn’t do this, and just let the clocks go their separate ways, we might cause serious problems to systems like GPS software that depend on super-super-accurate time-keeping.
So tonight, the official clocks will show 23:59:60 before rolling over to tomorrow, and everything is in its right place. Don’t worry if you forget to sync your watch. You’ll just be a second early everywhere tomorrow.
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