Shortest Day vs. Earliest Night

Posted by jns on 21 December 2009

I am always happy to celebrate the decision of our sun to return to a higher point in our northern sky, a decision it routinely takes about this time of year: 21 December. It seems so delightful that the days seem to start getting longer immediately it makes the decision.

And then, whenever the topic comes up, as it certainly has today, I always make a pest of myself by pointing out my favorite astronomical fact for those of us in the northern hemisphere: although the winter solstice marks the shortest amount of daylight in the year, the earliest sunset of the year actually happened three weeks ago, on 7 December. The last time I mentioned it in this space (in 2006) I didn’t have any really good, clear explanation to offer and I don’t this time, either. I posted a few links previously — look at them if you feel yourself the intrepid explorer of orbital dynamics — but for now we’ll just wave our hands and say it’s because of the tilt of the Earth, and the fact that it’s roughly spherical (at least, I think this would not happen if the Earth were cylindrical along its axis of rotation).

But anyway, the point I always make is this: by the time we get to the solstice, the sunset is already getting noticeably later and psychologically (to me, at least, since I rarely encounter sunrise) this makes us feel the day is getting longer at a rather brisk pace once the solstice is passed. The reason for that sensation, of course, is that it started three weeks ago. (Have fun verifying this for yourself with the NOAA Sunrise/Sunset calendar.)

For those who key on the sunrise to judge the length of their day–I’m sorry, but the latest sunrise falls on 7 January.