What is a Leap Second? Why Do We Use It?

Leap second

What is a Leap Second? Why Do We Use It?

One June 30, 2015 NASA will add a leap second to our time. So, instead of 23:59:59 it will be 23:59:60. You might think, just 1 second won’t do anything considerable to us, but you thought wrong. There are plenty of reasons why you need to understand what a leap second can do to us.

What is a Leap Second

UTC is the time standard that is used to determine the local times in time zones worldwide. It is based on the combined output of the highly precise atomic clocks – the reason for the statistical time scale called International Atomic Time.

Why a Leap Second

A leap second is the one second adjustment that is occasionally applied to the Coordinated Universal Time to keep the time of the day close to the mean solar time. Without the correction, the time from the earth’s rotation drifts ways from the atomic time because of the irregularities in the earth’s rate of rotation.

This wouldn’t be an issue if we still use sun dials and measure time using the rays of the sun as we won’t get accurate measurements of it anyway. However, at some point someone decided that the highest point int the sky is would be the day and the lowest would be the night – that’s when we have the accurate measurements of time. Each hour is divided into 60 minutes and each minutes is divided into 60 seconds.

Now that we have a precise measurement of time Atomic clocks ensure that it is as accurate as ever. Atomic clocks use an atom for a clock instead of the Earth’s motion around the sun. This means that the time scale is smaller and instead of 1 unit of time being a year, the oscillation is shorter than a second.

The earth does not revolve around the Sun in a steady pace, it wobbles. This wobbling affects the time we have on earth. The earth’s rotation differs from year to year. This is the reason why sometimes you feel that time is shorter at some days, and longer at the other.

The job of deciding when to add a leap second relies on the scientists at International Earth Rotation and Reference Systems Service (IERS). They determine the speed of the earth’s rotation through radio telescopes around the world and use a quasar as a reference point. This process is further explained in this video:

Can We Subtract A Second?

A leap second is not just adding 1 second to the UTC, scientists can also subtract – but so far this is only achieved in theories. All leap seconds are positive and given the slowing of the Earth’s rotation, we’re unlikely to see a reverse in time even for just a second.

Is it Necessary

A one second addition to our time is simply too accurate for most cases. This is according to Matsakis, who is an Astronomer himself. “One second is too coarse for accurate Astronomy” “Radio astronomers go to IRES to know the earth’s precise rotation instead of clocks. “As for mariners the, of course use GPS to navigate, which has no leap seconds.”

So the main point of adding a second to the UTC is to make sure that we as humans percieve the highest point the sun is to the sky as noon. However, even the keepers of UTC seem t doubt that leap second matters.

“There is this this false correlation between the sun and time,” says Elisa Felicitas Arias, the director of the time department at the Bureau International des Poids et Mésures—the supervisors of the world’s time standard. Without the leap second, she says, “One day you will have breakfast at noon, but not me—this will happen in thousands and thousands of years.”, she added.

How a Leap Second Affect Us

A second added to our time wouldn’t affect our daily lives right? Wrong. Because as more systems rely on technology, the risk of something bad that will happen is more probable when a leap second is inserted.

For instance, a computer cannot simply adjust for the leap second. Many computer programs and systems weren’t designed to adapt to leap seconds as some programmers who developed it didn’t consider that an additional second will affect the program.

The previous leap second which was inserted in June 2012, most of the websites were down. Gawker went down, Reddit went down, and most of the Linux Servers went down. Some airlines caused computer problems which also caused a lot of delayed f lights.

Google has a way of dealing with the leap second through “leap smear”. Google’s servers will split the leap second into two millisecond fragments then smear them over the preceding day.

The US stock markets will shut down a few minutes before the scheduled leap second as a precautionary measure just in case the system crashes.

As you can see, just a second added to our time could cause tremendous problems.

The leap second will happen at midnight UTC. This means that it will happen at different local times around the world: 5pm PDT, 8pm EDT, 1am BST, and 2am CEST


A leap second is an adjustment we make to simply have an accurate measurement of time. We need the accuracy of time so that we can cope up with the ever changing nature of earth.

But as you can see in the statement above, adding 1 second to our time causes more harm than good. Is it possible to abolish it?

Well, if the leap second is abolished, our noon might not be exactly accurate as it was before. However, our ancestors have lived and died without leap seconds, does it really matter now? By the time the earth becomes really out of sync, we might already have found a solution for a better time keeping technology anyway.

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