What is Leap Second? Why June 30 Will Be 1 Second Longer



Everyone longs for more time in life. Well, you get one second extra on 30th June this year! A leap second will be added on Tuesday, June 30, 2015 to account for slowing down Earth’s rotation.

According to Wikipedia, a leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time, or UT1. According to UTC, a day is 86400 seconds long i.e. 60 seconds per minute. But a mean solar day (according to UT1) i.e. the time taken by planet Earth to complete one rotation about its axis is slightly (about 2 milliseconds) longer than UTC day. This is due slowing down of Earth’s rotation primarily because of tidal friction between Earth and its moon.

To adjust this difference caused by accumulated 2 milliseconds, extra seconds are added to UTC. These additional seconds are called leap seconds. The first leap second was added on 30 June 1972. Since then, leap seconds have occurred at an average rate of one per 19 months.

Leap second is of two types: positive and negative. A positive leap second is added either on June 30 or December 31 between 23:59:59 and 00:00:00 of the next day. With the leap second on June 30, clock will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. A negative leap second would hold back 23:59:59 of last day of selected month. Hence 23:59:58 second would be followed directly by 00:00:00 of the next day. Negative leap seconds have never been used till now.

UTC clock screenshot from https://time.gov/ on 30 June 2012 leap second.
UTC clock screenshot from https://time.gov/ on 30 June 2012 leap second.

Since Earth’s rotation is affected by climatic disturbances and geological events, leap seconds are highly unpredictable. Atmospheric variations due to El Niño can cause Earth’s rotation to slow down. This increases the length of day by 1 millisecond. Variations in atmosphere, large water bodies, mountains and inner core of the Earth also affect its rotation considerably. International Earth Rotation and Reference Systems Service (IERS), therefore, decides upon inserting of leap second six months before the chosen date.

Also Read: Will the Leap Second on June 30 Break the Internet?

Last leap second was added on 30th June, 2012. The leap second on 30th June 2015 will be the fourth added since 2000. Chopo Ma, a geophysicist at NASA’s Goddard Space Flight Center and a member of the directing board of the International Earth Rotation and Reference Systems Service says, “The modeling of the Earth predicts that more and more leap seconds will be called for in the long-term, but we can’t say that one will be needed every year.”

NASA uses Very-long-baseline interferometry (VLBI) technique to study Earth’s rotation. In this signals from radio sources like quasars are received by radio telescopes on the Earth. VLBI is used in spacecraft tracking, astrometry, mapping movements of tectonic plates and other geodetic studies.

Leap second added in 2012 caused technical problems to many sites including Reddit, LinkedIn, Mozilla and others. Web server could not handle the extra second. Several proposals have been made to abolish leap second insertion but no such decision is expected in the near future.

Stay tune with fossBytes for more science and space news.

Kriti Kushwaha

Kriti Kushwaha

Kriti is an aspiring blogger and Electronics and Communication Engineering student from NIT-H who loves poetry and McD's burger, of course.
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