On Jul 3, 2012, at 1:54 PM, Valdis.Kletnieks@vt.edu wrote:
On Tue, 03 Jul 2012 21:49:40, Peter Lothberg said:
Leapseconds can be both positive and negative, but up to now, the earth has only slowed down, so we have added seconds.
That's what many people believe, but it's not exactly right. Leap seconds are added for the exact same reason leap days are - the earth's rotation isn't a clean multiple of the year. We know we need to stick in an entire leap day every 4 years or so, then add the 400 hack to get it closer. At that point, it's *really* close, to the point where just shimming in a second every once in a while is enough to get it back in sync.
IIRC, isn't it: Add a leap day every 4 years. Exception: If the year ends in 00, do not add a leap day. (an exception seemingly glossed over in the thread so far) Exception to the exception: If the year is a multiple of 400, add a leap day. (so called 400 hack) With that set of rules, we get close enough to only fudge by a second here and there. Owen