Galileo Then and Now: A Review Essay

The authors of these books about Galileo and the science of his age follow paths that diverge in interesting and striking ways, but they all agree about Galileo’s towering achievements in astronomy and physics. An instrument, the telescope, changed the world and compelled us to rethink our place in the universe. Galileo had eyes that were prepared to see new things and a hand that enabled him to depict what he saw. He was not only a gifted observer, but also an exceptional draftsman, which enabled him to discover what others had failed to see or lacked the ability to record. The telescope, invented around 1590 in Italy, was a crude device that enlarged four or five times and was little more than a plaything. Galileo had the brains to see its potential. He also had the good fortune of having access to the best lenses in Europe, those that were made on the island of Murano, just off Venice and to the present day the capital of glasswork. Without the unknown and unsung craftsmen who made excellent lenses, Galileo would not have been able to improve the telescope and render it capable of magnifying fifteen times, which is required to see the features of the celestial bodies that he recorded. Nor would he have achieved lasting fame as the Columbus of a new heaven. With his new instrument, Galileo made eight discoveries, all of them confirmed except the last, whose correct interpretation was beyond his ken. First, Galileo saw that the Moon has mountains and valleys and, hence, that it resembles the Earth. This was exciting news: if the Moon is like the Earth, then it might be inhabited! Second, innumerable stars popped out of the sky, and untold worlds were suddenly and unexpectedly revealed. Third, the Milky Way, which looks like a white band in the sky, turned out to be a vast number of small stars that are invisible to the naked eye. Fourth, the Moon has “earthshine” for the same reason that we have “moonlight,” only brighter because the Earth is four times as big as the Moon. The fifth discovery was even more sensational, as Galileo loudly proclaimed, for it made him the first person to observe new satellites. They were four in number and they went around Jupiter. Such a discovery had not been anticipated, even in the wildest dreams of philosophers or astronomers. Galileo named them, “Medicean stars,” in honor of the ruling family of Tuscany where he was born and where he soon hoped to be recalled. Sixth, Venus was revealed as having phases, an observation that proved that it orbited around the Sun, not the Earth. Seventh, even the Sun held a surprise: its face is covered with spots. The eighth, and the only problematic of Galileo’s discoveries, was what he took to be two satellites revolving around Saturn. They were troublesome because they changed shape and occasionally even disappeared. Galileo was baffled and made no secret of his embarrassment. What he had observed were what we now know to be the rings of Saturn that are sometimes seen edgewise, when they are hard to detect, and sometimes slanted when they can be identified with a more powerful telescope than the one he had. This was only achieved by Christiaan Huygens several years later.

The three authors also concur in celebrating Galileo’s achievements in physics and especially his discovery that all bodies fall at the same speed regardless of their weight. This was historically important because it led Newton to realize that new laws of motion were required to explain why this should be the case. The story that Galileo dropped balls from the Leaning Tower of Pisa is probably apocryphal, but he showed great ingenuity in devising experiments with rolling balls along an inclined plane. He carefully measured the distance they traveled and the time it took. The outcome was the law...