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Galileo and the 400th Anniversary of the Telescope

by Bob Joseph


Portrait of Galileo in his forties by Dominico Robusti (National Maritime Museum of London)

Four hundred years ago, Galileo Galilei looked at the heavens through a small eight-power telescope and initiated an astronomical revolution. He immediately made several surprising discoveries that contributed to the demise of the Earth-centered cosmology of Aristotle and Ptolemy that had dominated Western thought for two millennia. This 400th anniversary of Galileo's first use of the telescope for astronomy is part of the justification for choosing this year as the International Year of Astronomy.

In 1609, Galileo held the chair of mathematics at the University of Padua, then the leading Italian university. He was 45 years old. He taught astronomy, geometry, and military engineering, had become widely known as a brilliant lecturer, and was famous for his research into the physics of motion.

While visiting Venice (just a few miles from Padua) in July 1609, Galileo heard about an optical instrument that had been invented by a Dutch spectacle-maker, Hans Lippershey. This "Dutch perspective glass" was probably only a three-power telescope. Galileo went back to Padua, worked out the optical theory of a simple two-lens telescope, taught himself how to grind and polish lenses, and by August returned to Venice with an eight-power telescope for the city fathers. The military advantages for the powerful Venetian navy were of course obvious, and they rewarded him by doubling his salary and giving him a lifetime appointment at the university.

Galileo went back to Padua and became the first person to use a telescope for astronomy (except, apparently, for the Englishman Thomas Harriot, who did not publish his telescopic discoveries). Galileo built improved telescopes of 20- and 30-power. By December 1609, he had discovered that the Moon is not the perfectly smooth sphere Aristotle had claimed, but was covered by mountains and craters. He discovered hundreds of new stars in familiar constellations, and most importantly, he discovered the four inner satellites of Jupiter. These made much more plausible the idea advocated 65 years earlier by Nicolas Copernicus (1473-1543) that all the planets orbit the Sun and that the Moon is a satellite of Earth. In contrast, the Earth-centered model had the Moon, the Sun, the planets, and all the stars orbiting the fixed and unmoving Earth once a day.

Galileo saw the scientific significance of these discoveries but also realized that he could use them for personal advancement. He was a great self-promoter, and by March 1610 he had rushed into print a little book describing his discoveries. Meanwhile, he asked the ruling Medici family in Florence for permission to dedicate the book to Cosimo II, then the grand duke of Tuscany, and to name the four moons of Jupiter the Medicean stars (now called the Galilean satellites). This led to a series of fascinating negotiations. The Medici court had a difficult time making a decision, since it was not completely clear that Galileo's discoveries were genuine. Galileo went to Florence to demonstrate his results, and they finally agreed to Galileo's dedication. Galileo had also requested an appointment to the Medicean Court, and he was granted the title of chief philosopher and mathematician to the grand duke. In September 1610 he moved to Florence at a salary twice his professorial salary in Padua--and with no teaching responsibilities.

In the autumn of 1610, Galileo made another critical discovery using the telescope: that Venus shows a full range of phases just like those of the Moon. This is simply not possible if both the Sun and Venus orbit Earth, but can be understood the same way the lunar phases are understood if Venus is orbiting the Sun. This was another major argument in his defense of Copernicus' Sun-centered model of the heavens.

Galileo's greatest scientific contributions were his research into the theory of motion of projectiles and falling bodies. However, his demonstration of the power of the telescope for penetrating the mysteries of the heavens has dominated the development of astronomy for the past 400 years and continues to do so even today, as groups in both Europe and the United States propose to build optical/infrared telescopes larger than the 10-meter telescopes that currently define the state of the art.