Leap second | Wikipedia audio article
Channel: wikipedia tts
Date: 14/12/2018

This is an audio version of the Wikipedia Article:
Leap second


00:03:34 1 History
00:07:50 2 Insertion of leap seconds
00:10:16 3 Slowing rotation of the Earth
00:12:21 4 Proposal to abolish leap seconds
00:22:25 4.1 Examples of problems associated with the leap second
00:28:23 4.2 Workarounds for leap second problems
00:30:08 5 See also



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SUMMARY
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A leap second is a one-second adjustment that is occasionally applied to civil time Coordinated Universal Time (UTC) to keep it close to the mean solar time at Greenwich, in spite of the Earth's rotation slowdown and irregularities. UTC was introduced on 1972 January 1st, initially with a 10 second lag behind International Atomic Time (TAI). Since that date, 27 leap seconds have been inserted, the most recent on December 31, 2016 at 23:59:60 UTC, so in 2018, UTC lags behind TAI by an offset of 37 seconds.The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, uses the international system (SI) definition of the second. The UTC second has been calibrated with atomic clock on the duration of the Earth's mean day of the astronomical year 1900. Because the rotation of the Earth has since further slowed down, the duration of today's mean solar day is longer (by roughly 0.001 seconds) than 24 SI hours (86,400 SI seconds). UTC would step ahead of solar time and need adjustment even if the Earth's rotation remained constant in the future. Therefore, if the UTC day were defined as precisely 86,400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The point on the Earth's equator where the sun culminates at 12:00:00 UTC would wander to the East by some 300 m each year. The leap second compensates for this drift, by occasionally scheduling a UTC day with 86,401 or (in principle) 86,399 SI seconds.
When it occurs, a positive leap second is inserted between second 23:59:59 of a chosen UTC calendar date and second 00:00:00 of the following date. The definition of UTC states that the last day of December and June are preferred, with the last day of March or September as second preference, and the last day of any other month as third preference. All leap seconds (as of 2017) have been scheduled for either June 30 or December 31. The extra second is displayed on UTC clocks as 23:59:60. On clocks that display local time tied to UTC, the leap second may be inserted at the end of some other hour (or half-hour or quarter-hour), depending on the local time zone. A negative leap second would suppress second 23:59:59 of the last day of a chosen month, so that second 23:59:58 of that date would be followed immediately by second 00:00:00 of the following date. Since the introduction of leap seconds, the mean solar day has outpaced UTC only for very brief periods, and has not triggered a negative leap second.
Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), when needed to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.