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Today Is Leap Day, But This Year’s Leap Second Isn’t Until June

Today is leap day, the extra day inserted into the calender every four years (usually) to prevent seasonal shift. So what? Nothing you didn’t already know. But isn’t a day a pretty big unit of time, in the grand scheme of things? It’s amazing that shifting things by an entire day can keep everything in working order. Well, it actually can’t, which is part of the reasons we have leap seconds, and you’re in luck, because we’re getting one of those this year on June 30th at 23:59:60 UTC

Much like the leap day is used to ensure that the calender lines up with the seasons, the leap second is used to make sure that Coordinated Universal Time (UTC) stays aligned with the solar day. Given that a second is much less likely to affect your schedule, however, you might not be aware of it.

The leap second has been around since 1972 and adheres to a certain set of rules.

  • Leap seconds always occur at 23:59:60 UTC on either December 31st or July 30th. 1972 was the first and last time a leap second was added on both dates of the same year.
  • A leap second occurs at the exact same time around the globe. For instance, leap seconds occur at 27:59:60 EST
  • There is no set pattern for the insertion of leap seconds. Instead, it occurs any time that the difference between UTC and UT1 gets close to 0.6 seconds.
  • It’s possible to have negative leap seconds, but we haven’t used those yet.
  • Their placement is currently controlled by the International Earth Rotation and Reference Systems Service.

Ok, buy why leap seconds? As it turns out, the measurement of time — especially small units like seconds — is a little bit complicated, partly due to the fact that it’s done a lot of changing. For instance, a second used to be measured as 1/86400th of a solar days (or .0000155740740… solar days if you like decimals). When it became sufficently evident that the the Earth’s rotation isn’t exactly uniform, the length of a second was redefined to be measured as a fraction of the orbital year. Not long after that, it was finally narrowed down to the time it takes a caesium 133 atom to emit 9,192,631,770 oscillations of a radio wave. Needless to say, that’s completely independent from the length of the solar day.

On top all that modification to the length of a second, the length of a solar day isn’t static, whereas the length of a clock-day is. Due to variables like tidal friction, the length of the solar day slips by a few milliseconds here and there, so the leap second insertion is necessary ever now and then in order to keep things lined up the way we’re used to it.  The last leap second was in 2008 and while that happens to be a 4 year gap, that’s just a coincidence. In fact, this is the first and only time there’s been a 4 year gap between leap seconds. Besides, that leap second was in December, so it’s more like 3.5 years anyways.

Past leap seconds and irregular spacing aside, the moral of the story is that we can look forward to an increasingly rare leap second later this year. Set your alarms now, because there’s no predicting when one of these will come again. And remember not to blink; you might literally miss it.

(via Wikipedia)

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