Now that 2009 is behind us, the official "Year of Astronomy" is over. But is that going to stop us here? Of course not. In fact, let's pick up where the Year of Astronomy started it all, with Galileo.

It was in 1609 that Galileo revolutionized astronomy with his upgraded, improved version of a recently created tool called the telescope. It was department store quality by today's standards, but it opened up a whole new realm for those of us who are fascinated by the skies. Now we could see deeper into the heavens and resolve objects we didn't even know existed. Subsequent discoveries helped change the worldview of the western world forever.

One of his discoveries took place 400 years ago this week, and featured an object in the sky you can see this evening for yourself. On 7 January 1610, he fixed his tiny telescope on Jupiter and around it found "three fixed stars, totally invisible by their smallness."

That was an important discovery in itself, but what made this historic was the observations he made over the following week when, quite to his surprise, those tiny "stars" - and another newly discovered one - had moved! (His drawings are shown here.)

It appeared that these tiny "stars" were really going around Jupiter, as if they might be tiny jovian moons. You have to understand the paradigm at the time which dictated that such a thing just could not happen. The cosmic philosophy then was based on Aristotle's teachings that everything in the universe went around the Earth.

So for someone to boldly announce that there were tiny objects orbiting something other than the Earth was novel, and somewhat dangerous, to say the least. It brought to the forefront the question that if little moons could orbit a bigger object, why couldn't the earth orbit the sun?

But this wasn't the end of Galileo's battering of the aristotelian wall. He also found "imperfections" on the Moon - mountains and "seas" - which flew against the established view that all heavenly objects were unblemished. He also observed spots on the sun, further evidence of imperfections.

Later in 1610, and very importantly, he observed that the set of phases that Venus went through - similar to the phases of the Moon - could only be explained if Venus went around the sun, and not around us.

The year 1610 was a big year in astronomy, a great big year. This week alone, 400 years ago, was a monumental week. You can relive it yourself by going out in the next day or two and scoping out Jupiter setting in the southwest after sunset. See if you can pick out the Galilean satellites yourself. Studying more deeply this art of astronomy may ultimately change your worldview, as well.