Planetary Orbits #1: Prevailing Models

Tycho Brahe and Johann Kepler are one of the great pairs of science. The former collected immaculate data from his observations of the skies; which the latter then analyzed to come up with his famous laws of planetary motion. Very fairy tale like, right? Except that reality was far more complicated, as Kitty Ferguson describes in Tycho and Kepler, her awesome biography of the two men, their personal lives and their science.

This blog, the first in a four-part series, explains the prevailing astronomical views before Brahe and Kepler changed our understanding forever.

There was Ptolemy’s model, with the Earth at the center, and everything else orbiting the Earth. In this model, planets seemed to slow down and accelerate at times and even reverse direction: this is because we on Earth are moving in a circle while observing the other planets. Hence the concept of epicycles (circles within circles) was added to the model:

How did the epicycles help describe the observed motion?

“By adjusting the size, direction, and speed of the epicycles, astronomers could explain many of the irregularities observed in the way the planets, Sun and Moon move.”

Does this sound very convoluted? Why didn’t astronomers wonder what might cause such weird changes in both the direction and speed of motion?

1)      Because it was complicated enough to describe the motion of planets, so few wanted to venture into the “why” question.

2)     Blame Aristotle!

“(Aristotle had drawn) a line between physics and the mathematical sciences, including astronomy, in a way that could be interpreted to mean that astronomers need not search for Aristotelian “causes” for celestial motions.”

But why wasn’t Aristotle’s view questioned? Ah, because the Church had incorporated Aristotelian views; and the waters were now horribly muddied. Could one contradict any aspect of Aristotle without also (in effect) attacking the Church? But what about Aristotle’s view was so attractive to the Church in the first place? The fact that he had also declared that:

“Everything below the orbit of the Moon was subject to change, degradation, and decay, while the heavenly spheres beyond the Moon were a realm of unvarying, eternal perfection”.

This aligned with Christianity’s picture of “fallen, lost humanity on Earth and eternal, holy realms above”. Which is why his views were adopted by the Church.

The alternate model was by Copernicus that held the Sun at the center around which all the planets revolved. While the orbits weren’t as twisted as those in Ptolemy’s model, they weren’t perfectly circular either.

With neither model simple in its orbits, in the absence of any attempt to identify causes, it just seemed like a matter of preference (or geometric convenience) which model one believed: Ptolemy or Copernicus. But since Copernicus’ model didn’t align with Church doctrine whereas Ptolemy’s did, no points for guessing which view prevailed at that time…