# Johannes Kepler and Planetary Motion

Thales and Pythagoras suggested that the natural world could be understood. Aristotle dared to imagine what was beyond the Earth. Plato encouraged thought about the universe, even if he did take astronomy one step forward and two steps backward.

Copernicus followed in Ptolemy’s wake, devising the revolutionary heliocentric (sun-centric) model of the universe. Tycho Brahe may have (incorrectly) rejected that model, but he did make some of the most detailed night sky observations yet.

What’s more, by Johannes Kepler’s time, Tycho had cast doubt on the idea of uniform circular motion that had plagued astronomy for centuries.

At last, the world was ready for a more mathematical take on a question that had confounded philosophers, mathematicians, and classical astronomers alike: how do the planets truly move through space?

By standing on the shoulders of giants, Johannes Kepler was finally able to devise his three laws of planetary motion, which are still the leading mathematical theory today.

# Milankovitch and Climate

The Yugoslavian meteorologist Milutin Milankovitch is known for coming up with the idea of orbital forcing, also known as Milankovitch cycles. Orbital forcing is a fancy term for certain changes in Earth’s orbit, which are precession, obliquity, and eccentricity.

I’ve written about all three of these before, but here’s a brief overview:

Precession is the motion of Earth’s axis like a spinning top. Imagine the Earth’s day-by-day rotating motion as that of a top. What do tops also do? They wobble.

Although I’ve written about obliquity, I haven’t used the term before. It’s a fancy word for how the tilt of the Earth’s axis doesn’t stay at the same angle. It changes a bit over a very long period of time, ranging between 22° and 24°.

Eccentricity refers to the changing shape of Earth’s orbit. You might know that it’s not a perfect circle—it’s an ellipse, which I’ll talk about in more depth later. What you might not know is that how elliptical it is—that is, how far it is from being a perfect circle—also changes a bit over time.

These motions are all very well established in science today. We know that they each have an effect on Earth’s climate, and together they cause the ice ages. But Milutin Milankovitch, the scientist who first came up with the idea, had a lot going against him.

# Orbit and Climate

You have probably all heard of ice ages.

And no, I don’t mean the Ice Age movies…

Although, Ice Age is actually a pretty good example of what happens during a real-life ice age. I haven’t seen enough of the movies to really talk about how accurate they are, but I know there’s a lot of ice.

And a lot of breaking of ice.

These movies take place during a time when much of the northern and southern regions of the Earth were covered in glaciers. The world looked a lot like the satellite image up above. Whether mammoths and smilodons (sabre-toothed cats) actually lived then is another question entirely.

For the record, dinosaurs were definitely not still alive back then. Even deep under the ice. The quote from the third movie basically sums it up:

“I thought those guys were extinct!”

“Then that is one angry fossil…”

Yeah, they were extinct. But fiction can do whatever it wants.

But why do ice ages happen…and why isn’t the world covered in glaciers now?