Today is the great Leap Second Day, when an extra second is added to our clocks at midnight. For one odd moment, the official time will actually go from June 30 at 23:59:59 to 23:59:60 instead of directly to July 1 at 00:00:00.

The reason this is done is because the atomic clock standard we use has a very slightly different rate than the rotation-of-the-Earth based Coordinated Universal Time system. **To be clear: it’s not that the Earth is slowing down so much we have to add a second every couple of years!** It’s that

This has been planned for some time, and in fact I wrote about this in excruciating detail in January. Because there is simply no way I can top the brilliance of that post, I’ll simply repost it here. It’s a bit long, but that’s OK: you have an extra second today to read it.

*[Reposted from "Wait just a (leap) second" from January 23, 2012.]*

This summer will be a little bit longer than usual. A *tiny* little bit: one second, to be precise. The world’s official time keepers are adding a single second to the clocks at the end of June. This "leap second" is needed to keep various time scales in synch. It’s a bit of a pain and won’t really affect people much, but if it weren’t done things would get messy eventually.

This gets a bit detailed — which is where the fun is! — but in short it goes like this. We have two systems to measure time: our everyday one which is based on the rotation of the Earth, and a fancy-schmancy scientific and precise one based on vibrations of atoms. The two systems aren’t quite in synch, though, since the Earth counts a day as a tiny bit longer than the atomic clocks say it is. So every now and again, to get them back together, we add a leap second on to the atomic clocks. That holds them back for one second, and then things are lined up once again.

There. Nice and simple. But that’s spackling over all the really cool details! If you want a little more info, you can read the US Naval Observatory’s press release on this (PDF).

If you want the gory details, then sit back, and let me borrow a second of your time.

**Time after time**

There are lots of ways of keeping time. The basic unit *day* is based on the physical rotation of the Earth, and *year* is how long it takes to go around the Sun. But we need finer units than those! So we decided long ago to divide the day into 24 hours, and those into 60 minutes each, and *those* into 60 seconds each. In that case, there are 86,400 seconds in a day. OK, easy enough.

For most of us, that *is* enough. But scientists are picky (or "anal" if you want to be technical) and like to be more precise than that. And the thing is, the Earth is a bit of a sloppy time keeper. Tidal effects from the Sun and Moon, for example, slow it a bit. Other effects come in as well, changing the rate of the Earth’s rotation.

To account for this, in 1956 the International Committee for Weights and Measures made a decision: we’ll base the length of the second on the *year*, not the day. In fact, we’ll take the year as it was in the year 1900 (a nice round number, so why not) and say that the length of the second is exactly 1/31,556,925.9747 of the year as measured at the beginning of January 1900^{*}.

OK, fine. Now scientists have their ~~anal~~ precise definition, normal people have calendars, and we’re all happy, right?

*Right?*

**Sunrise, sunset**

Yeah. Not so much. Read More

This summer will be a little bit longer than usual. A *tiny* little bit: one second, to be precise. The world’s official time keepers are adding a single second to the clocks at the end of June. This "leap second" is needed to keep various time scales in synch. It’s a bit of a pain and won’t really affect people much, but if it weren’t done things would get messy eventually.

This gets a bit detailed — which is where the fun is! — but in short it goes like this. We have two systems to measure time: our everyday one which is based on the rotation of the Earth, and a fancy-schmancy scientific and precise one based on vibrations of atoms. The two systems aren’t quite in synch, though, since the Earth counts a day as a tiny bit longer than the atomic clocks say it is. So every now and again, to get them back together, we add a leap second on to the atomic clocks. That holds them back for one second, and then things are lined up once again.

There. Nice and simple. But that’s spackling over all the really cool details! If you want a little more info, you can read the US Naval Observatory’s press release on this (PDF).

If you want the gory details, then sit back, and let me borrow a second of your time.

**Time after time**

There are lots of ways of keeping time. The basic unit *day* is based on the physical rotation of the Earth, and *year* is how long it takes to go around the Sun. But we need finer units than those! So we decided long ago to divide the day into 24 hours, and those into 60 minutes each, and *those* into 60 seconds each. In that case, there are 86,400 seconds in a day. OK, easy enough.

For most of us, that *is* enough. But scientists are picky (or "anal" if you want to be technical) and like to be more precise than that. And the thing is, the Earth is a bit of a sloppy time keeper. Tidal effects from the Sun and Moon, for example, slow it a bit. Other effects come in as well, changing the rate of the Earth’s rotation.

To account for this, in 1956 the International Committee for Weights and Measures made a decision: we’ll base the length of the second on the *year*, not the day. In fact, we’ll take the year as it was in the year 1900 (a nice round number, so why not) and say that the length of the second is exactly 1/31,556,925.9747 of the year as measured at the beginning of January 1900^{*}.

OK, fine. Now scientists have their ~~anal~~ precise definition, normal people have calendars, and we’re all happy, right?

*Right?*

**Sunrise, sunset**

Yeah. Not so much. Read More