Galileo Gallilei - The Paramount Astronomer of the 17th Century

The 17th Century was something of a coming out party for science in general. For the first time in the history of the world, physics, astronomy and much of mathematics went from mythological musings to rigorous disciplines. Perhaps most important for astronomy were the contributions of Galileo Galilei.

Galileo Galilei (1564-1642) is often credited for inventing the astronomical telescope. Of course, he was simply one of the first to use the already invented telescope observe the heavens. After hearing of the telescope, he constructed his own three power version in 1609, and with that very primitive instrument discerned for the first time craters and mountains on the moon. That Galileo recognized such features (anyone can see the crater Tycho with his naked eye) should hardly be a surprise, but it was only the beginning of the uproar Galileo caused with the Catholic Church, which believed in a strictly Ptolemaic viewpoint that all of the heavenly bodies were perfect, featureless crystal spheres, and that the earth was the center of the Universe. It really is hard to understand how one can look at the moon, with its many dark maria, which form "the man in the moon," and regard it as a perfect crystal sphere, but recall that this was in the same socio-political climate which apparently allowed for very few records of comets or even the spectacular Crab Supernova in 1054, which was so bright it was visible in the day for weeks.

Undaunted, Galileo by April of 1610, had observed the phases of Venus, moons of Jupiter and spots on the Sun, which he published in his pamphlet, "The Starry Messenger." All of these phenomena flew in the face of the Ptolemaic view of the Universe. First, sunspots are both imperfect, and are transient---as the sun rotates, they move across the disc of the sun; also they wax and wane on a timescale of days to weeks. The Ptolemaic believed that all heavenly bodies were "immutable," and sunspots certainly are not that. Second, Galileo observed that Jupiter appeared to have its very own planets (now called the Galilean satellites), which destroyed the notion that all heavenly bodies orbit the earth. Last, the phases of Venus could only be explained if the sun were at the center of the "Universe," as shown below.

The phases of Venus, explained only by having the sun at the center of the Solar System. (R. Martin, UIUC)

So, Galileo had disproven, by any scientific standard, the Ptolemaic (and Catholic) mythology of the heavens.

The church obviously disapproved of Galileo's findings, but for the twenty or so years that followed, it resisted action, and described Galileo's work in much the same way the had dealt with Copernicus's, that their "theories" were esoteric mathematical schemes which, though possibly consistent with observations, were only harmless exercises in mathematics, and nothing to take seriously.

Galileo, however, did not want his work to be relegated to such status, and worked diligently to produce his classic volume, "Dialogues on the Great World Systems" in 1632. To reduce his exposure to prosecution, Galileo carefully structured the prose as a discussion of the merits of the both the Ptolemaic and Copernican systems, without specifically endorsing either. However, the scientific evidence he had gathered and included was so overwhelming that no sensible person could prefer the Ptolemaic scheme. Though a clever guise, the method was viewed by the church as a thinly veiled slap in the face. Upon threat of torture by the Church, Galileo recanted and lived out his life under house arrest until his death nine years later.

The church's draconic actions, which bitterly ended both the life and brilliant scientific career of Galileo, would be a Pyrrhic victory. Galileo's work quickly became the basis for all future astronomy, immediately including the work of Newton, Huygens and Cassini. Galileo's Universe is now taught to every elementary student as wrote fact, and the man himeself is firmly ensconced as one of the great scientists in all of history.

Two curiosities: 1. Only in the last few years has the rightly absolved Galileo (after long since acknowledging his view of the Solar System as correct), 2. Why do we call him by his first name??