Sunrise, Sunset

I hit the street at least half an hour later than usual this morning. A monstrous (but very welcome) thunderstorm woke me in the wee hours (sometime after 4 a.m.), and I went back to sleep hoping in vain that it might still be raining at 6. Alas! no, and when I finally got up around 7, I had to hustle.

It was pleasantly cool (low 70s F.), and, though it was a bit muggy, at least the streets hadn’t started to steam. Luckily the cloud cover hadn’t entirely broken up, so my route was still mostly shady even though the sun had gotten quite high. As I dodged the occasional puddle or blown-down limb and kept a wary eye on the sun as it fought through the clouds, I thought about sunrises and sunsets and a little-remarked phenomenon regarding them.

This year the summer solstice fell on June 20, the earliest date since 1896. As can be seen from the charts on this Web page, the date (on average, balanced by leap years) will continue to shift back until 2100. In general the backward progression is arrested every hundred years, since a century year is not a leap year, but 2000, being a century year divisible by 400, was a leap year, so the progression continued. An interesting factoid but not the one I was thinking about.

No, what interests me is that, although the days do start getting shorter after the summer solstice and longer after the winter solstice, the change in length affects sunrise and sunset unevenly. I first noticed this with regard to the winter solstice. You might expect that the sun would begin to rise earlier each morning after the solstice, but in fact it does not. On the contrary, it actually rises later for as much as a month afterward (the phenomenon is more pronounced the closer you are to the equator). For example, at my latitude, the sun rose at 6:44 on December 22, 2007. It continued to rise later until January 15, when it rose at 6:50. It did not rise as early as 6:44 again until January 30, and it was February 1 before it rose at 6:43. The day is instead lengthening at the other end. Similarly, after the summer solstice, sunrise times do start getting later, but sunsets are also later till well into July, though the disparity is not nearly as pronounced in the summer.

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