The revolution was not his alone. The idea was actually an ancient one, and other scientists had embraced it along the way. But it took Galileo and the telescope he built to prove the truth to the masses: Earth is not the center of the universe.
This year marks the 400th anniversary of the year Galileo turned his first crude telescope to the heavens. Through it, he observed spots on the sun and shadow patterns proving that the moon had mountains and valleys. These visible "imperfections" helped overturn thousands of years of traditional belief that everything in the heavens must be smooth, perfect and unchanging.
Galileo also watched Venus over many months, observing phases that proved it was actually orbiting the sun and not, as had previously been believed, orbiting Earth. He observed Jupiter night after night, discovering that it was always accompanied by four "stars" that clearly orbited the planet. We now know these "stars" to be Jupiter's four largest moons, and their existence offered definitive proof that Earth was not the center of everything.
Revolutionary change is never easy. Clear as the case may seem to us now, many of Galileo's contemporaries put up fierce resistance. Some refused his invitations to look through the telescope for fear of what they would see. The pope summoned Galileo to Rome, where an inquisition found him "vehemently suspect of heresy." He lived the rest of his life under house arrest.
Time and the passage of generations gradually weakened resistance to Galileo's observations, and new discoveries strengthened his case. Today, we know not only that Earth is a planet orbiting the sun, but that our sun is just one among more than 100 billion stars orbiting in the vast Milky Way galaxy, and that our galaxy, in turn, is only one of about 100 billion galaxies in the observable universe. In total, there are as many stars in the universe -- and very likely, as many planets -- as there are grains of sand on all the beaches on Earth combined.
The United Nations has declared 2009 the International Year of Astronomy, hoping to provide millions of people with the opportunity to learn more about the universe and about the discoveries of Galileo and others. This is a good thing. But astronomy is not just about science, and Galileo's revolution was not just about knowing Earth's physical place in the universe. It also was about human perspective -- our cosmic perspective -- and about how we should understand our place and purpose in the universe.
One need not be religious to see that a cosmic perspective gives some universal meaning to our lives. We may be only a tiny part of a vast universe, but we are here, and as a species, we have accomplished great things. We have created staggeringly beautiful works of art and music, we have performed acts of love and generosity that make even the most cynical among us quake with emotion, and we have developed mathematics and science that have enabled us to learn our place in the universe.
These are achievements of consequence. But the cosmic perspective also should teach us some humility, because the central lesson of Galileo's discoveries is that we humans are no more central to the universe than our planet or star. Future generations and alien civilizations may enjoy our human creations, but no one will come running to our rescue if we choose to destroy rather than to create.
Sadly, this lesson in humility seems not to have taken hold, despite the 400 years we have had to absorb it. Nearly everyone is now aware that we are not the center of the universe. But emotionally and behaviorally, our species still acts as though the whole of creation somehow revolves around each of us personally. How else can we explain tyrants and dictators? Or religious fanatics who believe that their God actually wants them to kill those who think differently? And before you let yourself off the hook, ask yourself honestly if you don't at least sometimes think that those who are poorer, sicker or otherwise less fortunate than you are also somehow a bit less central to the universe than you are.
As we think about science during the International Year of Astronomy, let's also show that we can finally absorb the lesson that we are not the center of the universe. This year, try extending a little more kindness to your fellow human beings, in recognition that we are all equally important. Try to demonstrate an understanding of the fact that we all share the same small planet by taking a better care of it. And perhaps most important, especially as we confront a time of crisis both for the economy and for international peace and security, remember that we must create our own legacy.
We can continue to act as though we are the center of the universe, but in that case we will suffer the consequences of our ignorance. Or we can develop a true cosmic perspective and set our civilization on a course to a better future for all, a future that will someday take our descendants to the stars.
Galileo once said, "You cannot teach a man anything; you can only help him find it within himself." In this 400th anniversary year of Galileo's greatest discoveries, let's hope that we can finally find their meaning within ourselves.
Jeffrey Bennett is an astrophysicist and the author of numerous books including "Beyond UFOs" and the children's book "Max Goes to Jupiter."