What Good Is a Telescope?

This month's IYA theme is telescopes. In fact, the reason this year is IYA is that 2009 marks the 400th anniversary of Galileo's first use of the astronomical telescope. So what? Why should you care?
I'm going to answer that question in three parts, each of which will go live in three consecutive days. So consider this the first part, but by no means the entire story. Check back for more tomorrow and the day after.
The first part of this story centers around the power of astronomy. The power of astronomy is the power to predict the future. That is to say, we all have enough astronomical knowledge to know that the sun will rise and set each day. We can plan around this and it gives us power. If we didn't have that knowledge, we wouldn't have the power to set our own schedules or even know when the best times to sleep or work would be. In earlier times, the power to predict the future was just as important. And that future was laid out in the stars. If you could know where the planet Venus would be 5 months in the future, that meant power. And if you predicted it incorrectly, that was even worse.
To understand the importance of telescopes, let's take a journey back in time to the early 17th century. At that time, the ruling view of the universe was the geocentric model articulated by Ptolemy and Aristotle before him. From this point of view, the sun, the moon, the planets, and the stars all orbited Earth in perfect spheres. This was the "onion" universe: the earth was at the center; the moon orbited in the sphere closest to the earth (like the layer of onion closest the center); Mercury and Venus orbited the earth just past the moon, each in their own layer; the sun was in the next layer out; followed by Mars; Jupiter; Saturn; and then the stars were the outside layer, all being part of one sphere that encircled all other spheres of orbit. In this model, the movement of celestial bodies was perfect and circular.
There was a bit of problem with this model; as all keen sky observers know, the planets don't follow a straight path in a single direction across the sky; occasionally, they seem to go backward in their path and then forward again, making a loop. How could this be explained in a model of spheres? The idea of epicycles sprung forth. Epicycles were orbits within orbits; that is to say, a planet like Mars would be moving in its own circular path while circling Earth.
This model of the universe was mostly accurate and allowed early astronomers to predict the positions of the planets más o menos (more or less, as we say in Spanish). Since the Catholic Church was one of the great powers of the day, The Church made sure that this perfect model was the one that people understood. God put Earth at the center of the universe (and by extension, man), and all other celestial bodies orbited in their perfect spheres. The Church could predict the future through this model of the universe and that gave The Church power.

Stay tuned for Part 2 of our telescope talk!