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Winter solstice marks shortest day

E-mail|Link|Comments () December 21, 2009 12:47 PM

By Andrew Ryan, Globe Staff

(Joanne Rathe/Globe Staff)

The sun slipped to its annual nadir in the sky today at exactly 12:47 p.m., marking the winter solstice in the northern hemisphere.

Boston will cling to just over nine hours of daylight today in what will be the shortest day of the year. The days will now begin to slowly grow longer: one minute more daylight over the next week; 30 minutes more over the next month; three hours more by mid March. Come the summer solstice in June, the sun will shine more than six hours longer than today, the first day of winter.

"It is actually the day when the angle of the sun is as its minimum," said Dani LeBlanc, a producer and educator at the Charles Hayden Planetarium at the Museum of Science. "Since the equinox, the sun has been getting lower and lower in the sky."

The earth is tilted at a 23.5-degree angle and the northern hemisphere is currently leaning at its farthest point away from the sun. But the distance is not what accounts for the waning daylight or winter cold. It's the angle, forcing much of sunlight to bounce off the earth and leaving the ground relatively cold.

"With the snow on the ground, you sort of have vicious circles of coldness," LeBlanc said. "Snow reflects even more sunlight and doesn't allow it to warm the earth."

For warmth, think of Trelew, a small city in the Argentine Patagonia which has roughly the same latitude as Boston in the southern hemisphere. The temperature in Trelew today is expected to hit 86 degrees as the sun shines for 15 hours and 24 minutes.

To learn more, the Charles Hayden Planetarium has the show "Winterlight: Stars and Symbols of the Solstice," which run through January 3.