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Genoa, Italy (1625 - 1712)
Giovanni Domenico Cassini was an Italian mathematician, astronomer, engineer, and astrologer. Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669, professor of astronomy at the University of Bologna and became, in 1671, director of the Paris Observatory. Along with Robert Hooke, Cassini is given credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Cassini was the first to observe four of Saturn's moons, which he called Sidera Lodoicea. Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.

Galileo: Frequently Asked Questions
by Dr. Jim Lattis, University of Wisconsin-Madison

Please click on an item to reveal the answer.

  • No. In fact, Galileo published his method for solar observing, so we know how he did it, and it is a safe method still used today. Rather than looking through the eyepiece of his telescope, Galileo projected the sunlight onto a screen placed a few feet behind his telescope's eyepiece and safely examined the projected image. Not only is this safe (if done without putting the eye in the path of the sunlight), but it allowed him to trace the outlines of sunspots directly onto the paper. Galileo would have been well aware of the perils of looking at the Sun even without a telescope. Moreover, the power of “burning” lenses to start fires was well known in his time; so it is unlikely that he would have risked his vision observing the Sun at the eyepiece. Like many people, Galileo lost his vision in his later years, probably as a direct result of aging, not foolhardy observing.

  • Not really. Galileo's telescopes, and other telescopes for some years afterwards, did not clearly reveal the rings as we are used to seeing them. What they showed was that Saturn had an odd shape, with what seemed to be extensions on each side. He didn't know what to make of these odd shapes, and the problem was complicated by the fact that they seem to come and go. (This is because our view of the tilted rings can go from wide to almost invisibly narrow in a few years as Earth and Saturn orbit the Sun.) Only some time later, in 1655, did Christian Huygens, in the Netherlands, propose that the observations could be explained by a ring around the planet.

  • No. To the best of our knowledge, the telescope was invented in the Netherlands by 1608 and was being sold across northern Europe by 1609. Although he had never actually seen one, Galileo read a rough description of the device in the summer of 1609 and was able to build one. Galileo recognized that an improved telescope could be more than a curiosity or toy and turned it into an instrument of great military, commercial, and scientific value. His improved telescopes led him to his famous astronomical discoveries.

  • No. Galileo tells us that evenbefore his telescopic discoveries he had already concluded that the Copernican theory (of a heliocentric solar system in which Earth is just one of the planets) was preferable to the ancient Aristotelian-Ptolemaic system (with the Earth stationary at the center of the cosmos and everything else circling around it.) But in his Copernican conviction, Galileo was very unusual. With absolutely no evidence for its truth, the Copernican theory was favored mostly by those who appreciated its mathematical harmonies or its calculational advantages. The vast majority of astronomers preferred to stick with the ancient geocentric models, not only owing to force of tradition, but also because they could not imagine the Earth sailing through space with three distinct motions, none of which we could feel at all, all the while keeping the Moon in tow as well.

    When Galileo showed that Jupiter has moons, he showed that the Earth was not the only center of motion. And the phases of Venus showed that without any doubt Venus orbits not the Earth but the Sun. These things were not evidence that the Copernican system was true, because although consistent with the Copernican theory, they could be accounted for with other theories (like that of Tycho Brahe). But they were utterly irreconcilable with the Ptolemaic-Aristotelian theory. Galileo's discoveries thus overthrew what had been the predominant cosmology since classical antiquity, and forced consideration of the alternatives.

  • No. Galileo got into trouble with the Inquisition in 1633, decades after his telescopic discoveries. In fact, his telescopic work had been confirmed and praised by Church astronomers, among others, soon after he published Starry Messenger in 1610. But in 1632, Galileo published his Dialog on the Two Great World Systems. In that book Galileo openly advocates the truth of the Copernican system (or so the Inquisition concluded), which he had earlier (in 1616) been ordered to consider only hypothetically. Thus, although there were many complicating factors, it was Galileo's defiance of the Church's attempt to control intellectual inquiry that began his troubles, not his astronomical observations.

  • No. Torture, in those days, and for quite some time to follow, was an accepted method for obtaining confessions, not only by the Inquisition but by Protestant thought police and secular authorities as well. It would have come as no news to Galileo that the Inquisition might use torture, but he was nevertheless formally notified of this fact as part of his trial. In the end, Galileo saw that he could not change the minds of the pope and cardinals who judged him, so he exercised the better part of valor: accused of heresy, he accepted responsibility, acknowledged his errors, and recanted his teachings. In return, the Inquisition spared the aged scientist anything harsher than house arrest. (This, however, was bad enough, because it was a life sentence which forbade him to publish or teach.)

  • Yes. Among others, at least one of Galileo's colleagues at the University of Padua, the philosopher Cesare Cremonini, refused to look through Galileo's telescope at the rugged lunar surface. But such cases were very much the exception. Most people, and especially his fellow astronomers, were actually eager to get a look through a telescope, which was a rare opportunity in those days. However, many observers were skeptical at first that the sights revealed in the telescope were really in the sky and not some kind of reflections, flaws, or tricks in the instrument itself. As the knowledge of how to make and use good telescopes spread, with Galileo's encouragement, this attitude quickly subsided.

  • Maybe not. As knowledge of the telescope spread, Galileo was neither the first nor the only experimenter who improved and used the new instrument. There is evidence that Thomas Harriot in England, Simon Marius in Germany, and the Jesuit astronomers working with Christoph Clavius in Rome, among others, observed the Moon and Jupiter's satellites independently of and about the same time as Galileo's observations. Galileo, however, was the first into print with these epochal discoveries and was the one who realized and explained to the world what the discoveries revealed about the cosmos.

  • Not exactly. Telescopes were, of course, coveted by astronomers immediately. But early telescopes had many flaws that made them hard to use. The Netherlandish telescope design adopted by Galileo used for its eye lens a diverging (or “negative”) lens with a concave surface. This design provides an upright image, making it handy for military and maritime use, but the field of view is narrow and requires the observer's eye to be in just the right place. In 1611, the German astronomer Johann Kepler published a telescope design that used a converging (“positive”) lens, with a convex surface, for its eye lens. Although this design yields an inverted image, making it less useful for land and sea, it also provides a wider field of view and is more tolerant of eye placement. This Keplerian design rapidly became preferred by astronomers over the Galilean, which is rarely seen today. The Keplerian telescope is the true ancestor of all later astronomical refractors (telescopes that use lenses to collect light).

  • Maybe not! Galileo's discoveries were world-shaking without a doubt. Less heralded at the time was the publication of Johann Kepler's book New Astronomy, which appeared in 1609. In it, Kepler showed that the orbit of Mars is an ellipse (and probably the orbits of the other planets as well). It is hard today to understand how shocking that was to a world that took for granted that all celestial motions were perfect circles (including, by the way, Galileo, who never accepted Kepler's work). Perfect circles had been the unquestioned foundation of astronomy since the ancient Greeks invented mathematical astronomy more than 2000 years before Galileo and Kepler were born. To throw away celestial circles was one of the most revolutionary ideas a scientist has ever presented. How did Kepler settle on ellipses? He used years of celestial observations compiled by Tycho Brahe, who had never seen a telescope (Tycho died in 1600), and was able to show that observational data demanded that the orbit of Mars is an ellipse. Kepler would have done his work regardless of whether the telescope had been invented, and Newton could have used Kepler's work to formulate his Law of Universal Gravitation quite without any help from the telescope. Still, without a doubt Galileo's telescope is one of the roots of modern astronomy.

  • Galileo began his astronomical observations in late 1609. He rushed the earlier ones into print by March 1610 in a little book written in Latin entitled Sidereus nuncius (i.e. Starry Messenger). His most important observations published there were:

    a) The Moon has a rugged landscape containing mountains and valleys that look very much like landscape features on Earth. This contradicted the Aristotelian notion that all bodies beyond Earth are made of a perfect celestial substance: The Moon looked like it was made of the same stuff as Earth.

    b) The light of the Milky Way comes from the combined light of countless stars too dim to see with the human eye, but revealed by the telescope. Other parts of the sky also contain previously unknown stars and nebulous objects that are revealed by the telescope.

    c) Jupiter is orbited by four moons (Galileo called them planets), previously unknown, which show that Earth is not the sole center of motion in the cosmos (as was maintained by the Aristotelians). This made plausible the Copernican assertion that Earth orbits the Sun, and is in turn orbited by the Moon. After all, Jupiter's moons faithfully continue to orbit as their planet moves through space. In a political masterstroke, Galileo named these new objects the “Medicean planets” after the family of the Medici dukes of Tuscany. This won him the position he coveted as the duke's mathematician and philosopher, free to research and write without the burden of teaching.

    d) Galileo did not mention sunspots in Starry Messenger, but he must have found them soon after, because by the following year he was demonstrating them to others. He used the changing appearance of sunspots to argue against the Aristotelian notion that celestial bodies are perfect and changeless, and he measured the Sun's rate of rotation by tracking the spots across the solar disk.

    e) The phases of Venus also did not appear in Starry Messenger. But by late 1610 Galileo began circulating word of this discovery -- that Venus changed its appearance, going from more rounded to crescent shape, in a way that could only be explained if it was orbiting the Sun. This was the coup de grâce for the Aristotelian-Ptolemaic cosmos because it demonstrated irrefutably and “to the scandal of the philosophers” (as Jesuit mathematician and Galileo fan Gregory St. Vincent crowed) that Venus orbits the Sun, not the Earth. While not proof of the Copernican cosmos (because it also fit the intermediate theory proposed by Tycho Brahe, for example), the phases of Venus convinced even the most conservative astronomers that a complete rethinking of planetary theories was inevitable.




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