Precession, Seasons, and Tropics
(You need a questioner's category called "Parent trying
to answer kids' question, which is what I am.) As precession
wobbles the Earth's axis, switching winter and summer in the
northern and southern hemispheres every 13,000 years, there must be
a period in the middle where the Earth, as it swings around, comes
"broadside" to the sun, at which point there would be no seasons.
But there is a question/answer in the archives that says this is
not so. Why not?
Also, are the tropics of Cancer and Capricorn getting closer
together as a result of precession?
This is a very good question, and one I find hard to answer. I
believe that even though the Earth is wobbling like a top as it
precesses, the seasonal alteration is so gradual we do not notice
it. It takes about 26000 years for one precession cycle. I do not
think there is a sudden change in seasonal time because
of this reason. About ten years ago there was a lot of ballyhoo
about rapid season change because of a cosmic impact. For an object
to torque the Earth so that the equator would become the pole, it
would have to be at least as large as Mars. That is not in our future.
David H. Levy
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Update: June 2012