In effect, we stop civil time for a length of time exactly the same as that needed for the Earth to turn on its axis far enough to make sundials (if they were that accurate) agree with the atomic clocks to within one second. The decision whether to insert an occasional leap second, which happens roughly every few years, is made by the International Earth Rotation and Reference Systems Service, based in the Observatory of Paris, France.
The need to insert leap seconds into our calendar arises primarily because of the slowing down of the Earth's rotation due to the tides raised by the Moon on the Earth as both objects orbit the Sun. The length of the mean solar day is now about 2 milliseconds longer than it was in1820, when it was almost exactly 24 hours or 86,400 seconds long. Now, it is approximately 86,400.002 seconds. In addition, the world's clocks are so accurate that we can detect that the Earth does not spin at an exactly constant rate. The movements of the Earth's atmosphere, oceans and crust lead to further small changes in the rate of rotation, just as a spinning ice skater slows down if she puts her arms out or speeds up if she pulls them in again.
The leap second at the end of 2008 introduces the International Year of Astronomy 2009, which marks the 400th anniversary of Galileo's first use of the telescope to observe the heavens. There will be national and international events organized throughout the year to mark the occasion.
Issued jointly by the Island of Ireland's public observatories: Dunsink Observatory, Dublin; Armagh Observatory, Armagh; Imbusch Observatory, Galway; and Blackrock Castle Observatory, Cork.
See here for further information on the International Year of Astronomy, and here for the all-Ireland programme of activities. The all-Ireland point of contact is Professor Mike Redfern, NUI Galway; E-mail: mike.redfernnuigalway.ie.
For additional information on time and its definition in Ireland contact Dr David Malone, Hamilton Institute, NUI Maynooth, david.malonenuim.ie.
FOR FURTHER INFORMATION CONTACT: Mark Bailey or David Asher at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; mebarm.ac.uk; or djaarm.ac.uk.
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