Welcome to Winter!

I have been tired of winter driving for about a week now, and it hasn't even been winter. Today is the December solstice, the official first day of winter here in the northern hemisphere, and the shortest day of the year here.

Not a lot shorter. Today is only one second shorter than yesterday, and tomorrow will only be three seconds longer than today. That's the nature of an extremum, like the longest day: the rate of change is instantaneously zero, and is very small for a while on either side (assuming some continuity conditions which certainly apply to the sun's motion in the sky).

If today is the shortest day, wouldn't it make sense to add the planned leap second to today instead of at the end of the year?

Ha, ha, just kidding! Today is the shortest day in the sense of the length of time between sunrise and sunset. Adding a leap second will not change that. It only lengthens the day in the sense of the time between midnight and midnight. I guess to be clear, maybe we should say that today is the shortest daytime of the year.

So about the leap second. Turns out that this year we had both a leap day (Feb 29) and a leap second. The last time that happened was 1992. Leap days are added to keep the calendar year in approximate synch with the seasons (and thus, with the Earth's orbit around the sun). The leap second, by contrast, is added to keep the calendar day in approximate synch with the rotation of the Earth (and thus high noon).

Since the rate at which the rotation of the Earth changes (it is slowing down, at least in part due to tidal forces) is variable and somewhat unpredictable, the The International Earth Rotation and Reference System Service (IERS) has to make a decision every six months regarding whether or not we are going to need a leap second added. So the decision was made back in July that we would need a leap second at the end of December. Then in January, they will decide whether or not we'll need one at the end of June (December and June being the only months for leap-seconds). I'm guessing we won't need one right away again in June, considering that the last two were in 1998 and 2005.

The leap second is added at the end of the last day of December. Instead of going directly from 23:59:59 on 12-31-08 to 00:00:00 on 1-1-09, there will be an intervening time of 23:59:60. Weird. About as weird as having a Feb 29, I guess (or Feb 30).

Does that mean that when we are counting down to the new year, we need to count down that last second twice? Only if you are counting down in UTC (Universal Time). If you will be counting down in US Central Standard Time, like I will, the leap second occurs at 17:59:60. Therefore, unless you are planning a really long countdown, you won't have to count any second twice.

_{Image by Alaina Orwitz. See her art page.}