What Is A Leap Second?
For those of you who have no idea what that means, a leap second is an additional second that is added to Coordinated Universal Time (UTC) in order to synchronize atomic clocks with astronomical time -- give or take about 0.9 seconds.
In layman's terms, it means we add a second to the world time clock to synchronize our simulated time with that of Earth's rotation.
Why Do We Add Leap Seconds?
To be honest, if we didn't add a second here and there our simulated clocks would be inaccurate.
The reason for that is that Earth's rotation around its own axis gradually slows down over time. This is subtle, and so it takes a while for the change to happen. It's not like the Earth is going to stop spinning within the next few years -- heck, not even for centuries.
Atomic clocks, on the other hand, can infinitely count time without a hiccup. They'll retain the same speed for millions and millions of years. For all intents and purposes, they are more accurate than the Earth's rotation. Naturally, we have to adjust the atomic clocks to account for this change in the planet's rotation.
Hence, adding a leap second to the world clock.
How Often are Leap Seconds Added?
The last leap second was added back in 2012, at precisely 23:59:60 UTC on June 30, 2012. You probably didn't even notice, did you? That's because a second is infinitesimal in relation to our daily lives.
It's not the first time this has happened either, it's not even the second time. Since 1972, a total of 25 leap seconds has been added to the world clock. To break it down, that means the Earth's rotation has slowed down a total of 25 seconds since that time.
Now, you begin to see just how slow the change is occurring.
It begs the question, what does this mean? Are all days 25 seconds longer now than they used to be? The answer is no. Only the days on which the leap seconds have been added are 86,401 seconds instead of the usual 86,400.
Does This Change the UTC-TAI?
The difference between the UTC and the International Atomic Time (UTC-TAI) changes when a leap second is added. In June, 2015 when we add a new leap second -- and every time thereafter -- the UTC-TAI clock will be that much different.
In the case of the next leap second we add, the UTC-TAI will be 36 seconds different than the UTC.
Relatively speaking, this won't have much of a bearing on your life.
How Do We Know When a Leap Second Is Needed?
The International Earth Rotation and Reference System Service (IERS) -- which is located in Paris, France -- continuously observes the Earth's rotation and compares it to the current atomic time. If and when the difference between them grows to be more than 0.9 seconds, the IERS orders a leap second to be added to the world clock.
Can a Leap Second Cause Problems?
In your daily life, not likely. You won't even notice a difference when the change happens on June 30, 2015 -- or any time thereafter.
That said, a leap second can cause problems across the world.
For instance, in the stock market the second -- as small as it is -- may have sweeping implications for traders. Asian markets are planning to approach the time change as normal, allowing for trades even during that second that's being added. US markets, on the other hand, are going to ignore the second completely pausing during that time. It seems silly that such a small amount of time could cause issues, but in the fast-paced world of trading, a second makes all the difference.
In addition, computers running Linux -- which is based on the UNIX operating system -- may experience crashes and malfunctions during the time change. This is because the leap second hasn't yet been factored into the operating system for obvious reasons. The OS is still working during the time that the second is added. Since a lot of what computers do rely on the current time -- including anything relating to the internet -- the machine can get a bit fussy when it happens. To put it more simply, you're telling the computer to lie and computers cannot do that.
Linux has since been updated to reflect the addition of a leap second, but there's no guarantee there won't be problems. The event is just too rare to account for any underlying bugs or issues that may occur. The last leap second that was added caused quite a few complications for companies like Qantas Airways.
Just to clarify, these problems will not cause the world to stop. They are insignificant in terms of the greater picture, but they still exist.