Galileo's Proof

Galileo wrote the "Dialogue Concerning the Two Chief World Systems" to illuminate the differences between an Earth-centered or geocentric universe and a Sun-centered or heliocentric one. There were two observations through his telescope that supported the Copernican idea that the Earth indeed orbited the Sun. The first was the fact that Venus showed phases and got smaller and larger in the sky as it orbited the Sun. If Venus orbited Earth, its diameter would not grow and shrink in size (the Moon's didn't!). Venus was largest right before it was in the "new moon" phase of being backlit by the Sun. In this position it would be closest to the Earth. As it made its way around the Sun, it would reflect more sunlight on its surface until the half facing Earth was fully lit (just like a full moon). In this phase it would be much smaller from Earth's perspective.

His second observation was that Jupiter appeared to be a mini-solar system in its own right, with multiple moons orbiting a larger planet. Maybe the planets orbited the massive Sun, just like the moons orbited Jupiter.



If only Galileo could've seen what the four largest moons looked like up close!



They are now known as the Galilean moons, and from the upper left going clockwise are Io, Europa, Callisto, and Ganymede. To date there are 61 moons orbiting Jupiter.

It Spins!

If only Galileo could see these close up shots of the sunspots!

Galileo's Shocking Discovery

The imperfect surface of the moon was probably not as shocking to Galileo as the mutable surface of the Sun. Galileo was a proponent of the Copernican, heliocentric "universe," and even Copernicus stated that the Sun represented all that was Good and perhaps represented God himself. What were these curious blisters on the most perfect celestial body in the universe? And why did they move?

Galileo's First Glimpse

First off, I'll dispel the idea that Galileo invented the telescope; he merely improved upon its design. Unlike any human at the time, however, he chose to point the telescope up to the night sky and document what he saw. He saw that the moon was pockmarked by craters and had a surface that resembled Earth's crust. It was no longer a perfect* celestial body but a whole other world with imperfections. To give you a taste of the resolution of his lenses, here's a shot from a modern telescope (I highly recommend clicking on the image for a better view):



*Although details of the moon's surface could not be seen by the naked eye, the dark splotches of its surface could easily be discerned by any earthling. These splotches were explained by two different ideas: a) the moon was so close to Earth that it was "corrupted" by our sins or b) the moon reflected Earth's continents. (Now we think that the dark regions are three billion year old lava flows instigated by early, Everest-size asteroid impacts.)

What Galileo Would Have Wanted

I remember relishing the fact that I lived in "interesting times." I was born in 1969, the year we landed on the moon. I had hope about humanity's future, and knew that the key to our survival was an understanding of everything scientific and nothing superstitious or astrology-based. I now teach high school juniors and seniors about astronomy, and I want to spread the word to anyone who may not grasp where they are and what they are made of. Galileo published his ideas in Italian instead of the (scientific) language of Latin so that the public would have a chance to read his ideas. I hope to simplify complex ideas and bring them down to Earth so that anyone can understand the cosmos. Enjoy!