When Galileo heard about a spyglass invented in Holland that made distant objects seem near, he immediately grasped its potentialities for the furtherance of his own career. How much about its mode of construction he learnt from others and how much he worked out for himself is not quite clear. What is clear, however, is that his tested skill in instrument-making gave him an advantage over his competitors and a lead in telescope-making that he maintained over the crucial two-year period (1609–11) within which all the major astronomical discoveries within the reach of a telescope of this type were made by him. By August 1609 he could present the Doge of Venice with a roughly 9x telescope. Impressed by the military and commercial potential of the ‘perspicillum’, a grateful Venetian Senate promised to double his salary.
However, Galileo saw even greater potential for himself elsewhere. By January 1610 he had constructed a 20x instrument and had discovered Earth-like irregularities on the Moon, four satellites orbiting Jupiter, and stars beyond numbering that were invisible to the naked eye. Later in 1610 came moving sunspots, the phases of Venus and the oddly changing shape of Saturn. Galileo rushed his first discoveries into print, dedicating the Sidereal Messenger to his former pupil, now Cosimo II, Grand Duke of Tuscany, and dubbing Jupiter’s moons the ‘Medicean’ planets. These shrewd gestures gained him the position he had long coveted as Mathematician and Philosopher to the Grand Duke. His little book caused an instant sensation and made him Europe’s most famous astronomer.
His lunar observations, vividly chronicled in the Sidereus Nuncius, made the greatest impact, perhaps, on the general public. Patches of light and shadow could now be plausibly identified as Earth-like features. Prominent among them were what seemed to be lunar mountains, whose peaks could be seen as spots of light at lunar dawn while the lowlands below them remained in shadow. And the mountains’ own shadows shortened as the lunar noon approached, just as one would expect.
The discovery that four satellites circled Jupiter showed that, contrary to Aristotle’s doctrine of natural motion, there is more than one center of rotation in the planetary system. It also eliminated an objection to the Copernican claim that the Earth carried its Moon along as it circled the Sun.
The phases of Venus from crescent to full and back again became Galileo’s most potent argument against the older cosmology. If Venus and the Sun both orbit the
Earth, with Venus below the Sun, Venus should always show a crescent shape to Earth, if illuminated by the Sun. If, however, Venus and Earth both orbit the Sun, Venus
should run through the same sequence of phases as does our own moon.
Galileo’s contributions to astronomy were of two very different kinds. First were the stunning observational discoveries of 1609–11. These did not involve the calculational feats of his great predecessors, Ptolemy, Copernicus and Kepler. Galileo was fortunate to be the first to be enabled to see in the heavens what no one before him had seen; what made him a great astronomer was his ability to grasp the significance for cosmology of what he saw. Second was his furtherance of the case for the Copernican system. He did more than any other single person to bring about the displacement of the Earth from its privileged place at the cosmic center with the vast shift in world view that followed.
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