Thursday, December 22, 2005

Seasons Spinnig 'Round Again

Posted by Kirk
I actually wrote this up last year but the beautiful thing about the Earth is that it keep on spinning. As Harry Chapin used to sing:

All my life's a circle;
Sunrise and sundown;
Moon rolls thru the nighttime;
Till the daybreak comes around.

All my life's a circle;
But I can't tell you why;
Season's spinning round again;
The years keep rollin' by.

Last year someone asked me an interesting question. "Does the amount of darkness at the solstice change from year to year at a given latitude?" As far as I can tell, it doesn't. At least not in a scientifically significant way.

What is interesting though is that I came upon an another phenomenon. In our hemisphere, we all know that on or around December 21st the winter solstice arrives and we have the smallest amount of daylight in the year. The sun appears to stand still thus, "sol-stice." On this date the sun ceases to creep around the horizon and begins moving the other way.

Most people assume this means this is the day of the latest sunrise and earliest sunset. Here's the catch though, the earliest sunset is not on the solstice and neither is the latest sunrise! The date of the earliest sunset is actually December 9th (at least at my latitude) while the date of the latest sunrise is something like January 5th. The solstice falls in between and it is neither the day with the latest sunrise nor the earliest sun set.

It is, however, the day with the least total amount of light and the most darkness. These "days of darkness" between earliest sunset and latest sunrise change based on latitude. Here in Minnesota that means that we officially entered the days of darkness Wednesday, December 8th at sunset. The were then 12 days of darkness until the solstice which was day 13.

So, the good news is that the sun is now setting later and later starting today giving us more evening light. (Though technically we're talking a matter of seconds here so don't expect any visible change!) The bad news is that the sun will keep on rising later, giving us more morning darkness, until around January 4th!

You can dissect the nuts and bolts of why this phenomenon by reading this article by the astronomical applications department of the US naval observatory.