Sunday, December 24, 2006

A Long, Drawn-Out Solstice

The winter solstice this year came on about December 22. It was the shortest day of the year, but not by much. There’s a ten-day stretch in which the earliest sunsets and latest sunrises of the year are within a minute of each other.

What I find interesting is that, while these time markers are nearly stationary at the solstice, they are both actually going in the same direction--later. In Southeast Michigan we had our earliest sunset around December 10, at 5:02 PM. At the solstice, the sun has begun its upward journey, and sets at 5:05 PM. Our latest sunrise, however, will be on January 4, at 8:04 AM. At the solstice, it is still getting later, at 8:01 AM. Midway between those two dates falls the solstice.

At the other solstice in June, the same phenomenon occurs: earliest sunrise and latest sunset are both getting earlier. June 15 gives us our earliest sunrise, at 4:57 AM, and the latest sunset comes on June 27, at 8:16 PM. Midway between these dates is the longest day of the year, on June 21.

It all has to do with the curvature of the earth and how the sun’s rays reach around the horizon. I don’t know enough geometry to figure it out for myself, so I depend upon the U.S. Navel Observatory to provide me with a chart of sun times. If you want to find out what these times are for your location, go to their web site:

http://aa.usno.navy.mil/data/docs/RS_OneYear.html

2 comments:

Judith said...

It seems like 8:07 is not correct. But with daylight savings perhaps it is more accurate. It seems that its light until 10:00 in middle of the summer. I look forward to reading where you're growth in the wetland takes you.

Wetland said...

Oops. You're right. I forgot daylight saving time. Sunrise on the summer solstice is 5:57 AM and sunset is 9:15 PM, Eastern Daylight Time.