This is the first of what I plan to turn into an on going weekly series I am calling “Where we come from.” The plan is that every Wednesday I will write a post about something that fits into the theme of the blog, it could be a painting, or a sculpture, or scientific idea or a song or just about anything else that I want, heck I might even be open to suggestions from the comments. The key is that what ever I am posting about needs to have been influential in getting the world to where it is now. It needs to have shaped the world and made it possible to have the conversations we are having today. Without further adieu here is where we come from.

It is now common knowledge that the Moon goes around the Earth and the planets travel around the Sun, but it was once thought that everything goes around the Earth. In ancient Alexandria around 150 AD there was an astronomer named Ptolemy. His theory was that the Earth is at the center of the Universe and that everything we can see in the sky is moving around us on massive crystal spheres. We can’t see the sphere’s because they are transparent crystals and all the stars stay in the same place relative to each other because the spheres are moving and the stars are just embedded in the spheres. At first blush it is a very nice theory. The motion of the Sun and stars is explained, the earth is still and motionless, we could even predict when an eclipse. When it comes to planets though the elegance of the theory breaks down.

The planets in our solar system don’t move at the same rate as the stars seem to so each planet needed it’s own crystal sphere. Fine so instead of one crystal sphere holding everything we now need eight, one for the Sun, one for the moon, one for the stars, and one for each of the other five planet’s we knew about: Mercury, Venus, Mars, Jupiter, Saturn. It still would seem to work as a theory, just a slightly more complicated one. What this hasn’t taken into account yet is a pattern known as retrograde motion. During our trip around the Sun there is a point where it will look like planets are moving backwards. Why would something on a giant moving sphere suddenly start going the wrong way? Ptolemy’s solution was deferents and epicycles. He suggested that each planet is actually on a second moving sphere called an epicycle attached to the first one called a deferent that was needed to explain why the planets move at all. Why would five of the heavenly bodies have deferents and epicycles?

The planets made the model sloppy, it took away the beauty that it once had but at the end of the day all that matters when discussing a scientific theory is that it works, that it can make reliable predictions. Ptolemy’s epicycles did just that, and with the support of the church nobody challenged the model until Nicolaus Copernicus in 1543. Copernicus Offered the idea that the Sun is at the center of the solar system and that the planets go around the Sun. This what we now know to be the truth but in the 16th century it was just an easier way to do the math to figure out what is going on in the sky. The Heliocentric (Sun centered) model did not seem to have any more explanatory power that Ptolemy’s epicycle based model. This changed in 1610 when Galileo Galilei made several observations that could not be explained by epicycles.

In January of 1610 Galileo viewed Jupiter through a telescope and saw what he thought were four new stars. By watching how the new stars moved he discovered that they were actually going around Jupiter, this was the first time we found something that was clearly and visibly revolving around something other than the Earth. Later on September of the same year he observed Venus through a telescope and discovered that it has phases just like we see on the moon. The heliocentric model could explain why there are phases, on Venus the light is coming from the Sun at the center and illuminating only one side of the planet at a time. When Venus is close to us the light isn’t reflected in our direction, but when the planet is at an angle to us we see it more like a full moon. The geocentric model would have said this is impossible. In one year Copernicus’s idea went from a much easier way to do the math to a what was seen as a dangerous and radical new idea with growing support.

The path from here to there was not an easy one but now almost any one can tell you the moon goes around the Earth and the Earth goes around the Sun.