Many of us probably won’t notice, but Dec. 21 is the shortest day of the year and the first day of winter. It arrives at 4:21 a.m., the moment of the winter solstice, the exact time when our northern hemisphere leans farthest from the sun.
In that instance, autumn ended and winter began. From now on, the days will grow longer. Day length in Atlanta on Saturday will be only about 9 hours, 54 minutes, but a month from now we will have gained 21 minutes of daylight.
Day length will continue to increase until the summer solstice occurs on June 20 — the longest day of the year and the first day of summer, when we will have 4.5 hours more sunlight than Saturday.
Ancient people took keen notice of solstices. Many cultures worldwide held feasts and celebrated holidays around the winter solstice. Many cultures lit bonfires to strengthen the expiring sun.
For plants and animals — including humans — the day-to-day changes in the amount of daylight and darkness have profound importance. The changes help trigger major seasonal behaviors, such as reproduction, migration and hibernation in animals, and winter dormancy and spring blooming in plants.
Most creatures detect daylight changes through special cells called photoreceptors in their eyes and brains. Depending on the amount of daylight, the cells trigger — or limit — production of certain hormones. In turn, the hormones initiate — or turn off — the seasonal behaviors.
For many of Georgia’s year-round birds, increasing day length stimulates a rise in hormone levels that prompt the birds to sing and establish spring breeding territories. Even in early January, with the days gradually growing longer, cardinals, Carolina wrens and tufted titmice may start singing. Soon, house finches, mockingbirds, bluebirds and others will join in. At first, their melodies may not be as exuberant as in spring. Still, their early songs will brighten up a winter day.
Happy Holidays.
IN THE SKY: From David Dundee, Tellus Science Museum astronomer: The Ursid meteor shower peaks this weekend at about 15 meteors per hour — in the east after dark. The moon will be last quarter on Sunday. Venus is in the west just after sunset and sets three hours later. Mars and Jupiter (rising) are in the east at sunset. Saturn is in the west at sunset.
Charles Seabrook can be reached at charles.seabrook@yahoo.com.
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