The Reason for the Season

Our friends in the Door Peninsula Astronomical Society wear little buttons this time of year. The buttons  say “Axial tilt is the reason for the season.”   And that is absolutely correct if we are thinking about weather, and apparently many of us are.  Lately , shivering Crossroads visitors have lamented “Why it is so cold?”

When I ask students to speculate, they figure that we are just closer to the Sun in summer. But that guess is wrong by about two million miles. Astronomers tell us that in the summer we are about 93  1/2 million miles from the sun. Right now, we earthlings are a mere 91 1/2 million miles away from our special star.  So if we are  so much closer, why is it so cold in winter?

You can’t help but have noticed that in winter, days are short and nights are long. In summer, the reverse is true. So during short winter days, our region is heated for fewer hours.

Also, in winter, when the sun shines from low above the horizon, it must pass through many more miles of the earth’s atmosphere than when the rays beat done from directly overhead. In winter,  the already less potent heat and light are absorbed by the extra thickness of  atmosphere. On cloudy or snowy days, much (up to 95%) of the heat that actually reaches the earth is reflected into space.

All of this happens because of the tilt of the earth on its axis. At this time of year, the South Pole  is pointing towards the sun while the North Pole is experiencing round-the-clock-darkness. [That must make it tough on Santa’s elves.]

This year, December 21st is the Winter Solstice, and from then on, the days will get longer.  So we would expect that winter might let up a bit. Wrong! Due to the phenomenon called “lag of the seasons” the coldest days usually occur in January and February. It will take enormous amounts of energy over several months to warm up the soil, rocks and lakes enough for spring to happen.  So am I saying that February will be worse than this month?  I don’t predict weather, but I do predict that days will get longer.

During the shortest days of the year, Crossroads will host several season  programs.  This week, Naturalist Joan Wilkie will present holiday-themed Family Programs at 3:30.  And one December 24th at 2:00, the traditional lecture: “Greeting Card Birds” will feature the birds depicted on the greeting cards the naturalist receives in 2013 and the Crossroads Bird of the Year will be named. For the last three years, cardinals have won with chickadees a close second. This year? Join the naturalist and a sometimes surprisingly large (any more than a handful  of folks would be surprising) group in this discussion of local birds and their winter survival.


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