What Did Galileo Discover With His Telescope?
Galileo's telescopic discoveries: mountains on the Moon, the moons of Jupiter, the phases of Venus, the Milky Way's resolution into stars, and sunspots.
What Did Galileo Discover With His Telescope?
In the summer of 1609, Galileo Galilei, then a mathematics professor at the University of Padua, workeding on a problem that had nothing to do with astronomy. He was drafting a treatise on motion, tutoring private students in fortification, and selling the geometric compasses he had invented. A letter arrived from a former student in Paris describing a device that made distant objects appear closer. By the end of August, Galileo had built his own version of the device and had turned it on the heavens. The story of what he saw over the next two years is one of the great set-pieces of the history of science. This page looks at Galileo’s telescopic discoveries in the order in which he made them and published them: the craters and mountains of the Moon (late 1609); the moons of Jupiter, the resolution of the Milky Way into stars, and the unfamiliar “triple” appearance of Saturn (early 1610, in the Sidereus Nuncius); the phases of Venus, the strange shape of Saturn, and the sunspots (1610–1613, in the Letters on Sunspots and other works). The full biographical context is on the Galileo page, and the consequences of these discoveries for cosmology are part of the longer story of the telescope and the microscope. The way they supported the heliocentric theory is treated in the Heliocentrism and Astronomy and in Why Was Copernicus’s Theory Controversial?.
The Telescope in 1609
The first telescope was a Dutch invention of 1608. The spectacle-makers Hans Lippershey and Jacob Metius, in Middelburg, applied for a patent on a device consisting of a convex and a concave lens in a tube, which magnified distant objects by a factor of about three. The device was soon the talk of Europe. The Venetian ambassador, in a letter, and the Parisian savant, Jacques Badovere, in a published account, both sent news of the instrument to Italy in the summer of 1609. Galileo’s response was characteristically practical. He set about teaching himself the optics of lenses (which was not well understood at the time), worked out a theory of the magnifying power of the instrument, and built his first telescope in late June or early July 1609. He improved it rapidly, achieving magnifications of about eight, twenty, and eventually thirty. By November 1609 he had a telescope good enough to use on the heavens, and he began observing. The telescopes Galileo built were simple refracting telescopes: a convex objective lens at the far end of a tube, a concave eyepiece lens at the near end. The image was upright (the inverted image of astronomical telescopes, which use two convex lenses, came later, in the work of Kepler). The instruments had very narrow fields of view, were difficult to aim, and produced dim, blurry images. They were, however, vastly better than the naked eye. The details they revealed had never been seen before.
Mountains on the Moon
Galileo’s first celestial discovery was the Moon’s surface. He observed the Moon in late 1609 and published his findings in the Sidereus Nuncius in March 1610. The Aristotelian cosmology held that the Moon was a perfect sphere, made of the unchanging fifth element, without blemish. Galileo showed, by a series of careful observations, that this was false. The terminator — the line dividing the lit and unlit portions of the Moon — was jagged rather than smooth, and the shape of the jaggedness changed as the terminator moved across the lunar surface. Galileo interpreted the pattern as the shadows of mountains. He estimated the height of one of the lunar mountains at more than four miles, comparable to terrestrial mountains. The “seas” of the Moon (the dark, flat areas that he called maria) and the bright highlands were clearly distinct. The Moon, in short, was a world with a varied surface, not a polished crystal sphere. The discovery was, in some ways, the most fundamental of Galileo’s observations. It undermined the Aristotelian distinction between the perfect, unchanging heavens and the imperfect, changeable Earth. If the Moon had mountains, then either the heavens were not perfect, or the Earth was not imperfect. Galileo’s choice was clear: the heavens were not perfect. The implications were explored in How Did Copernicus Change Astronomy?.
The Moons of Jupiter
Galileo’s second discovery, the one that made him famous, was the moons of Jupiter. On 7 January 1610, he observed three small stars near Jupiter, arranged in a line. The next night, they were in different positions. Over the following weeks, Galileo observed that the four “stars” — he eventually identified four moons, not three — were orbiting Jupiter, in the same direction and roughly in the same plane, with periods ranging from about two days to about seventeen days. The discovery was announced in the Sidereus Nuncius. Galileo named the moons the “Medicean stars,” in honor of the Medici family, his hoped-for patrons. (They are now called Io, Europa, Ganymede, and Callisto, by the names assigned by the German astronomer Simon Marius, who claimed to have observed them independently, just before Galileo.) The political calculation paid off: in the summer of 1610 Galileo was appointed to a position at the Medici court in Florence. The scientific significance of the discovery was, however, greater than the political. Jupiter’s moons showed, in a single, easily observable case, that there was a center of motion in the universe other than the Earth. The Aristotelian system, in which the Earth was the unique center of all celestial motion, was simply refuted by the existence of these moons. The discovery also provided a useful clock: the regular eclipses of the moons of Jupiter, which Galileo tried to use to determine longitude at sea, became an important tool for cartography and navigation in the seventeenth century.
The Resolution of the Milky Way
A third discovery, also reported in the Sidereus Nuncius, was the resolution of the Milky Way. The faint, cloudy band that crosses the night sky had been a puzzle since antiquity. Aristotle had suggested that it was a kind of vapor, halfway between the Earth and the lunar sphere. Galileo turned his telescope on it and found that the cloud resolved into a swarm of countless faint stars, too small and too close together to be resolved by the naked eye. The stars varied in brightness and density from place to place. The “nebula” was a stellar phenomenon. This was not, in itself, a direct attack on Aristotelianism, but it was a discovery about the structure of the heavens, and it set the stage for later astronomical work on the distribution of stars. It also showed the power of the new instrument: it revealed an order in the heavens that had been hidden from view, and that could not be detected without it.
Saturn’s Strange Appearance
In July 1610, Galileo observed Saturn and was puzzled to find that it appeared not as a single disk, as Mars or Jupiter did, but as a triple object — a central body flanked by two smaller bodies. He announced the discovery in a cryptic anagram, lest someone else claim priority: “SMAISMRMILMEPOETALEUMIBUNENUGTTAURIAS” — which, when unscrambled, read, “Altissimum planetam tergeminum observavi,” “I have observed the highest planet in triple form.” What Galileo had seen, in his small telescope, were the rings of Saturn, then appearing as two separate objects flanking the planet, because his telescope could not resolve them as a continuous ring. He was mystified by the observation, and the problem was solved only by Christiaan Huygens in 1655, with a better telescope. Huygens identified the rings as a ring, a flat disk separated from the planet by a gap. Saturn’s strange appearance in 1610 was, for Galileo, an embarrassment: he never publicly retracted his “triple” claim, and he was later embarrassed when Saturn, in 1612, briefly appeared round, when the rings were edge-on to the Earth.
The Phases of Venus
In late 1610, Galileo observed Venus through his telescope and discovered that it showed phases, like the Moon. The discovery, which he announced in a letter to Giuliano de’ Medici in December 1610, was the most decisive telescopic argument for the Copernican system. The phases of Venus are predicted by both the Ptolemaic and the Copernican systems, but in different ways. In the Ptolemaic system, Venus orbits the Earth on an epicycle whose center is on the line between the Earth and the Sun. In this configuration, Venus can never be on the far side of the Sun from the Earth; it can only be between the Earth and the Sun, or off to one side. As a result, the phases of Venus in the Ptolemaic system can never be a “full” phase. The brightest Venus can ever be is a thin crescent, when it is between the Earth and the Sun and close to the Earth. In the Copernican system, by contrast, Venus orbits the Sun, on a smaller circle. As Venus moves around the Sun, it goes from being on the far side of the Sun (where it appears full) to being on the near side (where it appears as a crescent). It shows a complete set of phases, like the Moon. The Ptolemaic system cannot accommodate a “full” Venus; the Copernican system predicts it. Galileo observed that Venus did, in fact, show all phases. The full phase was difficult to catch, because Venus is on the far side of the Sun at the time, and it is lost in the Sun’s glare. But Galileo observed a sufficiently wide range of phases to rule out the Ptolemaic model. The Tychonic system, in which Venus orbits the Sun while the Sun orbits the Earth, also predicted the phases. So the discovery was decisive against the Ptolemaic system but did not, by itself, decide between Copernicus and Tycho. Galileo’s discussion of the phases, in the Dialogue Concerning the Two Chief World Systems (1632), was one of the most famous passages in the book.
The Sunspots
In the autumn of 1610, several observers independently observed dark spots on the Sun. The Jesuit astronomer Christopher Scheiner observed them in Ingolstadt and, in 1612, published a tract under the pseudonym “Apelles” arguing that the spots were small planets transiting in front of the Sun. Galileo, observing them from Padua and Florence, argued in his Istoria e dimostrazioni intorno alle macchie solari (1613), the Letters on Sunspots, that the spots were on the surface of the Sun itself, that they changed shape from day to day, and that they showed the Sun to be a variable, changeable body — again, not the perfect, unchanging sphere of Aristotelian cosmology. The dispute with Scheiner was one of the most public controversies of Galileo’s career. It was also significant for the history of science as an early example of the use of priority arguments in scientific publication. Galileo had been observing the spots since the autumn of 1610; Scheiner began observing them in 1611. Galileo published first, and he made a point of the priority. The dispute bleed into the controversy over the Copernican theory and the trial of 1633.
The Long-term Impact
The effect of Galileo’s telescopic discoveries was enormous. The Moon was a world with a surface. Jupiter had its own system of moons, showing that the Earth was not the unique center of motion. Venus showed phases that the Ptolemaic system could not explain. The Milky Way was a vast collection of stars, not a vapor. The Sun had spots, contradicting the doctrine of celestial perfection. The heavens, in short, were not the way Aristotle had said they were. The new instrument, in the hands of a brilliant and aggressive observer, had done what the old instruments could not: it had opened the heavens to direct investigation. The full impact of these discoveries on the heliocentric theory is treated in Heliocentrism and Astronomy. The way the discoveries eventually led to Galileo’s confrontation with the Church is told in Why Was Galileo Put on Trial?. And the broader history of the telescope as an instrument of discovery is surveyed in The Telescope and the Microscope.