It's Groundhog Day. The day when we try to figure out who's right about when the seasons start.

In the United States, people have come to believe that the seasons should be divided at the solstices and equinoxes (or "equinoctes", if you want to be true to Latin). It's an easy conclusion to reach. After all, those four points in the Earth's orbit are the only events in orbital mechanics marked on calendars. But people also tend to think that summer is when the sun is highest in the sky and winter is when it's lowest (when viewed from the northern hemisphere).

If you look at the red area of the picture, which depicts the American notion of seasons, you can see that spring and summer involve equal heights of the sun, as do autumn and winter. The azure area depicts a more sensible division of the seasons, if you want summer to be the highest time for the sun, and winter the lowest. (The sun actually appears to travel in a sort of figure-8 called the analemma, but the circle shape is less messy and it gives the correct height of the sun while making equal arcs represent approximately equal amounts of time.)

The Chinese calendar starts around February 6th for this reason. (It's a lunar-solar calendar, so leap months keep it from hitting the mark very accurately.) I believe that the Groundhog Day's myth about there being, or not being, 6 more weeks of winter derives from this question. It should be more like 7 weeks, of course, since February 2nd is a bit early, but then the holiday was invented by people that thought a groundhog could do better than them.



Groundhog Day


chuckle8's picture
chuckle8 4 years 5 weeks ago

I like it. My father, a construction worker, always said on July 4th the summer seemed halfway over.

I would like another slight adjustment, It seems the heating of the earth has a lag time in it. Instead of the height of the sun being the determining factor, I would like someone to use a little thermodynamics and say when the earth (in the northern hemisphere) is the warmest.

mathboy's picture
mathboy 4 years 3 weeks ago

There is a lag. I don't know when the peak is, but interestingly, there's also a lag in the maximum temperature during a day. The peak is when the rate of reradiation matches the rate of energy intake.

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