According to biographical notes made by a disciple, companion, and pupil of Galileo for the three years immediately preceding his death in 1642 and published a dozen years later:
[T]o the great confusion of all the philosophers, very many conclusions of Aristotle himself about the nature of motion, which had been theretofore held as most clear and indubitable, were convicted of falseness by means of experiments and by sound demonstrations and discourses; as among others, that the velocity of moving bodies of the same composition, unequal in weight, moving through the same medium, do not attain the proportion of their weights, as Aristotle assigned to them, but rather that they move with equal velocity, proving this by repeated experiments performed from the summit of the Campanile of Pisa, in the presence of all other teachers and philosophers and of all the students. (Holton, Physics, 51)
From these words grew the legend that Galileo dropped simultaneously from the Leaning Tower of Pisa iron cannon balls of different weights, and that they hit the ground simultaneously. The story could well be true. Galileo, however, never claimed credit for any such demonstration while he was alive—an unlikely omission had he really carried out the purported experiment.
The story serves to substantiate the myth that all scientific theories originate in experiment or observation. An alternative view to Galileo empiricist is Galileo theorist. He used logic rather than, or at least in addition to, experiment to examine and understand physical laws. In a dialog in one of his books attacking and demolishing Aristotelian physics, Galileo had his wise philosopher argue:
But without other experiences, by a short and conclusive demonstration, we can prove clearly that it is not true that a heavier moveable [body] is moved more swiftly than another, less heavy . . . then if we had two moveables whose natural speeds were unequal, it is evident that were we to connect the slower to the faster, the latter would be partly retarded by the slower, and this would be partly speeded up by the faster . . . But if this is so, and if it is also true that a large stone is moved with eight degrees of speed, for example, and a smaller one with four, then joining both together, their composite will be moved with a speed less than eight degrees. But the two stones joined together make a larger stone than the first one which was moved with eight degrees of speed; therefore this greater stone is moved less swiftly than the lesser one. But this is contrary to your assumption. So you see how, from the supposition that the heavier body is moved more swiftly than the less heavy, I conclude that the heavier moves less swiftly. (Holton, Physics, 81)
Galileo's thought experiment is more compelling than an attempt in the 1980s to repeat the supposed historical experiment. Unfortunately for pedagogues in search of simple classroom illustrations, the lighter ball dropped from the Leaning Tower hit the ground first! (The experimenter believed that he had released the balls simultaneously, but perhaps fatigue in his hand holding the heavier ball caused that hand to let go more slowly?)
Another legend has Galileo's study of pendulums stimulated by observing a hanging incense burner swinging back and forth in the Pisa Cathedral when he was a student, in 1583. Supposedly, Galileo noticed that each swing took the same time, whether the arc of the swing was large or small. This legend might also be true, although Galileo didn't develop his theory of pendulums until 1638, and the huge bronze incense burner in the central aisle now pointed to as Galileo's inspiration was installed after a fire severely damaged the interior of the cathedral in 1596.
in which he, Thomas, described the great orb of the heavens as "garnished with lights innumerable and reaching up in Sphaericall altitude without end" (Hetherington, Encyclopedia of Cosmology, 93-94, 176-77). His diagram depicts stars scattered at varying distances beyond the former boundary of the sphere of the stars and notes that the stars extend infinitely upwards. The diagram also illustrates continued obliviousness to another logical consequence of the Copernican model: Thomas accorded to the Sun a unique place at the center of the universe, even though the concept of a center loses all meaning in an infinite universe.
The idea of the Moon as a planet similar to the Earth would be dramatically emphasized by Galileo Galilei's telescopic discoveries beginning in 1610. He was born in 1564, the year of Shakespeare's birth and Michelangelo's death. His father wanted him to study medicine, which Galileo did at the University of Pisa. But he preferred mathematics and studied it with private tutors in Pisa and then at home in Florence. Soon he was giving private lessons and then was appointed to the mathematics chair at the University of Pisa when he was only 26 years old. Obstinate and argumentative, Galileo was unpopular with the other professors. His experiment involving cannon balls dropped from the Leaning Tower of Pisa—if it really occurred—was a public challenge to Aristotelian philosophers.
In 1592 Galileo left Tuscany to take up the chair of mathematics at the University of Padua, in the Republic of Venice. He would stay there 18 years. In 1609 rumor reached Galileo from Holland of a device using pieces of curved glass to make distant objects on the Earth appear near. He quickly constructed his own telescope and turned it to the heavens.
One of the first objects he viewed was the Moon. Large dark spots had been seen by many observers before Galileo, but his far more detailed telescopic observations more emphatically demanded the revolutionary conclusion that the Moon was not a smooth sphere, as Aristotelians had maintained, but was uneven and rough, like the Earth. Discernible features of the Moon—the so-called man in the moon—might be attributed by followers of Aristotle to vapors and mists terrestrial in origin or to lunar mountains and craters under a smooth, transparent covering material. But such ex post facto arguments were no match for Galileo's new telescopic observations.
In his Sidereus nuncius (Starry Messenger) of 1610, Galileo reported his observations of numerous small spots on the Moon. The boundary dividing the dark and light parts of the Moon a few days after new moon was not a uniform oval line (as it would have been for a smooth sphere) but was uneven and wavy. Within the illuminated region were a few bright spots. Galileo wrote: "The surface of the Moon is not smooth, uniform, and precisely spherical as a great number of philosophers believe it to be, but is uneven, rough, and full of cavities and prominences, being not unlike the face of the Earth, relieved by chains of mountains and deep valleys" (Drake, Discoveries, 31). The analogy to mountains on the Earth was further strengthened by Galileo's observation that the blackened parts of lunar spots were always directed
Figure 15.3: Frontispiece, Galileo Galilei, Sidereus nuncius, 1610. ASTRONOMICAL / MESSENGER / great and admirable sights / displayed to the gaze of everyone, but especially / PHILOSOPHERS and ASTRONOMERS, observed by / GALILEO GALILEO / Florentine patrician / University of Padua public mathematician / with a spyglass / devised by him, about the face of the Moon, countless fixed stars, / THE MILKY WAY, NEBULOUS STARS, / but especially / FOUR PLANETS / around the star JUPITER at unequal intervals, periods / with wonderful swiftness; by anyone / until this day unknown, / the author detected and decided / to name first / MEDICEAN STARS.
Image copyright History of Science Collections, University of Oklahoma Libraries.
toward the Sun, while the opposite side was bright, like a shining mountain peak.
Near new moon, the portion of the Moon shadowed from direct sunlight was slightly brightened. Ruling out an inherent and natural light of the Moon or reflected light from Venus, Galileo argued that the source of light was sunlight reflected from the Earth: earthshine. Thus the Earth was not devoid of light, but was similar to bodies such as the Moon and Venus.
Galileo's observations of satellites of Jupiter furnished another similarity between the Earth and a planet: both had moons circling them. Thus there was more than one center of motion, as the Copernican system asserted and the Ptolemaic system had denied. But much as Galileo wanted proof of the Copernican system, Tycho Brahe's planetary system (discussed in the following chapter), a compromise adopted by many influential Jesuit astronomers, also had more than one center of motion. Theories may be refuted by observation but not proved, because some other theory with the same observational consequences is always possible. Still, Galileo's observations strengthened the Copernican case.
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