Just about everyone knows that, in the northern hemisphere, the day with the least amount of daylight marks the winter solstice, which occurs on or about December 21. It is not, however, the day with the earliest sunset or the latest sunrise. The earliest sunset occurs on December 7, or, as it does this year, December 8.

Years ago, wondering why this was so, I called the Naval Observatory in DC. Someone there explained the reason for this to me. Basically the cause is that, because the earth is at perihelion in January, it is moving faster in its orbit than it is at aphelion, which occurs in July. It is related to something called the "Equation of Time."

Not many people know about the effect, so, when my mom said she had seen something about it in the paper this morning, I just had to find it.

Forecast: Sunnier Days Ahead

The effect is illustrated at Equation of Time, but it helps to have someone sit down and go through a slow explanation of how, because of the Earth's motion along its orbit and its proximity to the Sun now, it has to turn a little more than 360 degress from one day to the next so that the Sun is directly overhead at noon. If you compare noon as indicated by a sundial to noon as indicated by a clock, you will notice the disparity.

Every website shows or has a link to the chart that shows the daily discrepancy, but I can't find a site that really explains the effect simply, which is, believe it or not, easy to do.

It doesn't seem possible, but watchmakers have, for centuries, made watches that can compensate for the difference.

Mouvement d'horlogerie à quantième perpétuel comportant un mécanisme à équation de temps avec affichage

You won't see this at Wal*Mart.

If you haven't had enough, Google for "equation of time," or ask any astronomer.

This really is the best time of year. The sunsets are beautiful, and in the southeastern US, the dreadful humidity and mosquitos are blissfully gone. DC never looks handsomer than it does in December. All the museums and other buildings, like the Willard Hotel, are full of poinsettias and displays. I must get down to the Botanical Gardens.

By the way, my watch has stopped. Does anyone know what time it is?

[ December 04, 2005, 06:08 PM: Message edited by: twin58 ]

Time to get a new watch! wink

Twin, thanks. I have been curious about this too, and never was given an explanation for it. Although the linked explanation is technical, it makes sense. Anyway, I'm glad that the sunset is starting to get later. Although at this time, the difference in sunset times are seconds.

The sun never sets on old threads.

Good one twin58! The later the sunset the better, especially at this time of year, when it seems its always dark and cold around these parts, and up in Canada.

And does that explain why when I bend over I can see Uranus?

I am certainly ready to hybernate! It's already too cold for me and Winter has not officially started yet.Looking forward to longer days,soon!!
My favorite astronomical effect is the "Harvest Moon"

How does time factor into your GPS coordinates?

Time and GPS? I don't know.

The dean of the DC-area weather forecasters is Bob Ryan, from Buffalo, New York. He puts out an almanac every year. On November 30, 2005, the sun set at 4:47 p.m. On December 1, the sun set at 4:45 p.m. The sun set remains at 4:45 p.m all the way through December 12. On the 13th, the sun sets at 4:47 p.m again. That makes December 7 or so the day with the earliest sunset.

Meanwhile, the sunrise is getting later every day. On December 1, the sun rose at 7:07 a.m. On December 7, sunrise is at 7:13 a.m., and on December 13, sunrise is at 7:17 a.m. The sun rises at 7:27 a.m. from December 31 all the way through January 10. Split the difference to get the day with the latest sunrise as January 4 or January 5.

DC is at about 39 degrees north latitude.

[ December 04, 2005, 05:49 PM: Message edited by: twin58 ]

Tonite's "old moon in the new moon's arms, is pretty nice, especially with the evening star alongside right now. I'd like to see the Zodiacal Light and Aurora Borealis sometime, too. But if I ever wake up and see this in the sky, I think I'll just stay in bed!

And how many know that because of the earth's wobble in its axis tilt, that in about 10,000 years, its tilt will be a reverse from what it is now. As a result, winter in the northern hemisphere will begin in June, while summer begins in December.

So much for a white Christmas in Chicago THEN, eh?

MIB, if you're thinking of the precession cycle, I don't think you've got it right. It's usually described as affecting the timing of perihelion (when earth is nearest the sun), and aphelion (when earth is farthest away). Currently, the earth is nearest the sun in early January and that will change to July in 10,000 years but AFAIK, the timing of the solstices is not affected by that change.

If the solstices also changed, it would seem that it would have no effect on climate.

gmginsfo, thanks for posting the link to the analemma site again.

Yes, Venus and Mars are alright tonight, and Venus and the moon are even better. Both Venus and the moon have set by now in the east. Since Mars rises at about sunset, we are experiencing a "full" Mars.

I get up before dawn every day, and Saturday morning, I went out to see if I could see Mercury. It rose at 5:47 a.m. on Saturday, and the sun rose a little after 7:00 that morning. Mercury has to rise enough to get past the local haze, but no so much that the sky is light. I was able to see it between 6:00 a.m. and 6:15 a.m. Clouds rolled in and cut off the view.

Sirius is now setting at sunrise. A close second to my favorite astronomical effect is that Sirius is due south at midnight on New Year's Eve. It truly is a spectacle.

Just imagine that, 6,000 years ago, people studied the heavens constantly and kept records of just such observations.