My favorite astronomical effect.

[mahatmakanejeeves]

I wrote this back in 2005. Since then, Wikipedia has come along. The Equation of Time is mentioned in this week's Sky and Telescope, and it links to the Wikipedia article on the Equation of Time.

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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 did in 2003, 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, back in 2003, said she had seen something about it in the paper, I just had to find it.

Forecast: Sunnier Days Ahead

Sunday, December 7, 2003; Page B06

Buck up! Despite this weekend's wintry weather, there is hope.

Many people know that the start of winter and shortest day of the year is the winter solstice, usually Dec. 21 (it slips a day some years). Not widely recognized is that even though the 21st has the shortest day, it does not have the earliest sunset. The earliest sunset of the year is tomorrow, Dec. 8.

From here on out the afternoons get longer, and we can begin to revel in additional, growing light to guide our homeward commutes. This quirk results because the latest sunrise on the calendar is about Jan. 4, but the earliest sunset is about Dec. 8. The winter solstice of Dec. 21 is midway between these two dates.

So, no need to wait for Dec. 21 to herald the start of "longer" days.

GEORGE BROWN
Washington

The effect is illustrated at Equation of Time.

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.

The dean of the DC-area weather forecasters, Bob Ryan, from Buffalo, New York, puts out an almanac every year. Let's look at some times for sunsets and sunrises from his almanac. On November 30, 2005, the sun set at 4:47 p.m. On December 1, the sun set at 4:45 p.m. The sunset 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.

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?

[Igel]

1. In high school physics we covered this, sort of.

What do you expect, it was high school.

But ever since then, whenever I look at the clock and see it's noon (and not daylight savings time) I think, in quick succession, that it's about solar noon and then I wonder if the clock is fast or slow wrt solar noon.