Until I recently figured it out, I was becoming more and more irritated by the lecture course I've been watching called "Understanding the Universe: An Introduction to Astronomy." Geez, it seemed like grade-school stuff - especially the lecture devoted to how the seasons occur. Then I realized these first few lectures were devoted to "celestial sights that everyone can see." OK, we'll move on...eventually.
Regarding that "how the seasons occur" lecture, I sat through much of it with a certain amount of ennui. The professor, Alex Filippenko of the University of California, Berkeley, explained that studies of the graduates of big-league universities consistently show that these people - by a 25-to-2 margin, say that seasons are caused because the earth moves away from and closer to the sun. That's why Filippenko felt it necessary to devote a whole lecture to the topic. So, off we go, talking about how the earth is tilted 23 and a half degrees away from the plane of the solar system, so the northern hemisphere gets more energy in summer, blah, blah. (I thought everybody knew this.)
Anyway, at the end of the lecture, the professor raised a topic that I had "learned," but had pretty much forgotten: "precession." (It was the similar (but strangely different) precession of Mercury that so befuddled early 20th Century scientists - a befuddlement ended by Einstein.) For Earth, precession means that gravitational forces of the sun and moon, mostly, want to tip us over. But our planet's rotation resists this, and instead it just precesses - it sort of wobbles around like a top.
This wobble occurs over a 26,000-year period. This means that in half that time - 13,000 years - the earth will tilt the other way! The northern hemisphere will have summer in what used to be the winter, and winter in what used to be the summer! And 13,000 years later still, we'll be back to where we are today!
I find this so cool! The transition will be so slow as to be imperceptible over anyone's lifetime. But just imagine (if we're lucky) kids enjoying a cold winter day in July, 15,010 - "Look at the snow! Grab your sled!"