There are 86,400 seconds in 24 hours. But the speed of the earth's orbit is still not precise even with the added day.
Because the Earth slows down its revolving process by approximately two milliseconds per day, then an extra second needs to be added every five hundred years.
A leap second in July 2012, for instance, wreaked havoc on Reddit and a number of other websites including Gawker, Instagram, Pinterest and Netflix. These seconds are added at intervals ranging from half a year to seven year periods. They are added either on June 30th at the stroke of midnight or on December 31st of a given year. In an era of super-accurate atomic clocks, scientists discovered that the daily rotation of the earth isn't as predictable as previously thought.
According to the US Naval observatory, the new leap second will be instituted at 23 hours, 59 minutes and 59 seconds UTC on December 31, which corresponds to 6:59:59 PM Eastern Standard Time. The UTC is computed at the International Bureau of Weights and Measures in Paris. The bad news, according to Time and Date AS, A Norwegian company dedicated to the study of worldwide time and those who keep it, is that this messes up time-sensitive services such as air-traffic control systems.
The International Earth Rotation and Reference Systems Service (IERS) is the organisation which monitors the difference in the two time scales and calls for leap seconds to be inserted in or removed from UTC when necessary to keep them within 0.9 seconds of each other. Earlier, the rotation of Earth with respect to celestial bodies that accounted for the mean time.
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The reason for this is that the earth is slowing down.
Thus, the rotation of Terra slows compared to atomic clocks.
Atomic time is based on the regular, precise vibration of cesium atoms, which oscillate at precisely 9,192,631,770 times per second. For example, leap seconds were added every year from 1992 to 1995. So this year, we also get a "leap second", if you will. Unfortunately, discrepancy would eventually occur between the astronomical time, which is based on the length of one Earth day, and the atomic time, which is measured by about 400 atomic clocks around the world.
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To synchronize the world, timekeepers will add one second to the clocks on December 31.