On Jul 3, 2012, at 1:54 PM, valdis.kletni...@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