07 January 2008

Galileo and the Four Moons

On this date in 1610, Galileo Galilei discovered the four largest moons of Jupiter.

Well, to be honest, we aren't sure of the exact date he discovered them. As conscientious a note-taker as Galileo was, we can't quite pin down his first glimpse of Io, Europa, Ganymede and Callisto, but it was this date that he first let anyone else know about them. He wrote about them in a letter.

Although Galileo is still today given credit for discovery of the moons, he did not give them their lasting names. He originally named them after the great patrons the de' Medicis. Nothing curries favor with the boss like naming a planetary body after them. The four lasting names were chosen by Simon Marius, a figure of somewhat less lasting fame than Galileo, who claimed to have discovered the moons at the exact same time. He chose to name them for four of the many, many lovers of the Greek god Zeus (Jupiter in the Roman pantheon). The appropriateness of the names seemingly overcame Galileo's right of priority, and I have to admit I'm glad. I'm usually quite a fan of priority in naming conflicts (possibly the nerdiest of all conflicts, by the way) but I would much rather think of the possibility of life on Europa than ponder the frozen seas of Ferdinandipharus. Seriously. Ferdinandipharus.

Europa, whatever you call it, is one of the most promising places for life beyond Earth in our solar system. Though covered in ice, many researchers believe there may be a subsurface ocean of liquid water. This speculation has, of course, spawned volumes of science fiction, from Arthur C. Clarke's 2010 to the works of Caitlin Kiernan and Dan Simmons.

There were several important consequences to the discovery, whoever made it first, of the four moons. Galileo was a believer in Copernicus' theory that the Sun, not the Earth, was the center of the Universe. He famously and unfortunately paid the price for these beliefs, being basically imprisoned by the Inquisition and forced to recant. What he could not take back, though, was his discovery of the moons. They were the most powerful evidence yet that the Earth could not be the center of everything, as Ptolemy said it was. A few relatively lame arguments were made about the moons and Jupiter having incredibly close orbits, making it only seem like they were revolving around each other when in fact they were both revolving around the Earth, just together. It was a ridiculous theory, but one becomes more amenable to such things at the point of a sword, and geocentrism would remain a while longer.

Still, the discovery of the moons did more than deepen our understanding of our own solar system. It opened up the universe. Many astronomers of the time believed that all such major bodies in the universe had been found, and the discovery of the moons basically legitimized the use of the telescope for finding new things in the sky. It made our universe seem bigger, more mysterious, and more worthy of study than it had before, and we've never really stopped looking up since.

Tomorrow, Probability and Disaster

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