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What is a Leap Second ? & Why should you care?

Updated on February 8, 2015


A leap second is a second that is occasionally applied to Coordinated Universal Time (UTC) so that atomic clocks can be synchronized with solar (astronomical) time. Just to explain some of the terminology: Coordinated Universal Time (UTC) -- is the globally accepted time standard by which the world regulates regulates clocks and time. Atomic clocks - are the most accurate and stable timekeeping devices made by man. They are more accurate than the astronomical observations that our time scales used to rely on. Solar (astronomical) time -- is the passage of time that we perceive on Earth in regards to the passage of astronomical bodies such as the sun and moon.

Rationale Behind Leap Seconds

For centuries, solar (astronomical) time was has been the absolute standard in timekeeping and calender maintenance. However, slowly and surely scientists, astronomers, and other curious minds suspected that the number of seconds in a year were not constant year after year. (Why this happens will be elaborated on later, suffice it to say that the rotation of the Earth slows down a seconds every year.) With the rise of new technologies and modes of transportation that operate across multiple time zones; maintaining the consistency of seconds year after year became more of a priority.

Ever since the invention of the first accurate atomic clock in 1955 by Louis Essen, atomic clock technology has improved to the point where it is now the best man-made way to track time. This also meant that people were able to accurately determine how many seconds were lost between the years due to the slow down in the Earth's rotation.

Therefore, beginning in 1972, a one-second adjustment in the widely used UTC was implemented to bring the world clocks and others methods of time measurement in line with the supremely accurate atomic clocks. The one-second adjustment itself if implemented on either the end of December 31st or June 30th.

The UTC timezones as the apply to all countries across the world.
The UTC timezones as the apply to all countries across the world. | Source

Use of Leap Seconds Since 1972

Since the introduction of Leap Seconds in 1972, there have been a total of 25 leap seconds added to UTC. The last addition was made on June 30, 2012. Between 1972 and 2012, on average a new leap second has been added to Coordinated Universal Time every 18 months. Moreover, the organization that decides when and how to add a leap second is International Earth Rotation and Reference System Service (IERS) based in Paris, France. This organization will constantly monitor the rotation of the Earth on its axis. When it goes out of sync with International Atomic Time - TAI, they will order that a leap year be added to the UTC.

Irregular Spacing:

Although leap seconds have been added to UTC on average every 18 months; leap seconds are not added at regular intervals. Instead, leap seconds are added in a very irregular matter because the slow down of the earth's rotation is very irregular. The slow down in the Earth's rotation is faster during some years than others. Moreover, the IERS only decides whether or not to add a leap second to UTC 6 months in advance.

Possible Problems w/ The Leap Second:

Even though leap seconds are a de facto part of the international time and dating system, they do have their detractors. For example, many people in the computer science realm argue that leap seconds cause a lot of programming errors when trying to compute otherwise simple temporal problems. For example, to compute the exact difference between two dates in the julian calender, a program has to consult a table of leap seconds to make sure that it gets the correct answer. Believe it or not, problems like these cause a lot of problems for the websites, apps, and all the other digital products that we consume. For example, during the last addition of leap seconds which occurred on June 30, 2012, Reddit, Qantas Airways, Mozilla and various other sites running Linux went down. See the video below for more information on the debate over the use of leap seconds.

Debate on Use of Leap Seconds

The 2015 Leap Second: Is This The End?


This year on June 30, 2015, there will be the addition of another leap second as per the recommendation of the IERS. Many pundits fear that this particular leap year addition will cause widespread damage to the information technologies that we use today. Beyond the website problems that plagued many websites in 2012, tech companies posit that the GPS systems that airlines and truckers rely one may be affected.

n fact, companies such as tech giant google have taken steps far in advance to make sure that leap seconds do not affect their varied and complex systems.

Why The Earth's Rotation Slows

As mentioned before, the whole reason for adding leap seconds is because the rotation of the earth slows down a little bit every year or so. If left alone, clocks would go completely out of sync with the more accurate atomic clocks. So what causes this phenomenon of the Earth's rotation slowing down.

Well overwhelmingly, the biggest cause of this slowing down is tidal friction or "tidal acceleration." This is the process by which the gravitational pull of the moon exerts a force on the Earth that transfers angular momentum from the spin of the Earth to the orbit of the moon. That's a lot of technical talk so if you want a simpler explanation chock full of examples I suggest visiting this link: here

Other, less prominent causes of earth's slowing rotation include earthquakes or other large seismic events. These natural occurrences briefly change the Eartih's moment of inertia which "describes how mass is distributed throughout the Earth itself." For more elaboration on how this mechanism actually works please visit the same link that I had referenced before. Suffice it to say that seismic events generally only effect the Earth's spin only very slightly.


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