Even Ali Lost A Few

Galileo Galilei is sometimes called the father of modern science, and it is well known that he made a lot of enemies when he claimed that the Earth revolves around the sun and not vice versa. What most people don't know is that his best argument in favor of a moving Earth was wrong.

Galileo accumulated a wealth of evidence for a sun-centered solar system, but he believed that his strongest argument was his explanation of the tides. The rising and falling of the water level on the shores of Italy was mysterious to Galileo's contemporaries, but he explained it thus: Since the Earth spins on its own axis as it revolves around the sun, there are times when a particular part of the Earth (Italy, say) is moving in the same direction as the entire Earth in its orbit. Twelve hours later, Italy is moving in the opposite direction than the Earth as a whole, since it has spun around to the other side. In another twelve hours, Italy is back where it started, and the Earth's spin again carries it in the same direction that the Earth moves as it orbits the sun.

As the Earth spins, any part of the Earth alternatively moves with and then against the Earth's orbital motion. This change of velocity, Galileo argued, literally sloshes the Earth's oceans about, causing a high and low tide every twenty-four hours.

Well, not quite. Galileo's mistaken concept of inertia had misled him about the effect of the Earth's spin and orbit, and invoking the gravity of the moon to explain the tides did not occur to him. Perhaps we should be forgiving; an understanding of inertia and gravity sufficient to explain the tides was not really developed until Isaac Newton put his mind to it, roughly half a century later.

But why dwell on Galileo's failures? You can encounter one of his successes by observing Jupiter with binoculars this month. Jupiter is the brilliant white star-like object about half-way up in the sky, due south at sunset. Binoculars are all you need to make out its four largest moons, which were discovered by Galileo in 1610, and are appropiately named the "Galilean satellites."