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that would prove that Aristarchus, Copernicus and Kepler were all correct.

      

      Seeing Is Believing

      Born in Pisa on 15 February 1564, Galileo Galilei has often been referred to as the father of science, and indeed his claim to that title is founded on a staggeringly impressive track record. He may not have been the first to develop a scientific theory, or the first to conduct an experiment, or the first to observe nature, or even the first to prove the power of invention, but he was probably the first to excel at all of these, being a brilliant theorist, a master experimentalist, a meticulous observer and a skilled inventor.

      He demonstrated his multiple skills during his student years, when his mind wandered during a cathedral service and he noticed a swinging chandelier. He used his own pulse to measure the time of each swing and observed that the period for the back-and-forth cycle remained constant, even though the wide arc of the swing at the start of the service had faded to just a gentle sway by the end. Once home, he switched from observational to experimental mode and toyed with pendulums of different lengths and weights. He then used his experimental data to develop a theory that explained how the period of swing is independent of the angle of swing and of the weight of the bob, but depends only on the length of the pendulum. After pure research, Galileo switched into invention mode and collaborated on the development of the pulsilogia, a simple pendulum whose regular swinging allowed it to act as a timing device.

      In particular, the device could be used to measure a patient’s pulse rate, thereby reversing the roles in his original observation when he used his pulse to measure the period of the swinging lamp. He was studying to be a doctor at the time, but this was his one and only contribution to medicine. Subsequently he persuaded his father to allow him to abandon medicine and pursue a career in science.

      In addition to his undoubted intellect, Galileo’s success as a scientist would rely on his tremendous curiosity about the world and everything in it. He was well aware of his inquisitive nature and once exclaimed:‘When shall I cease from wondering?’

      This curiosity was coupled with a rebellious streak. He had no respect for authority, inasmuch as he did not accept that anything was true just because it had been stated by teachers, theologians or the ancient Greeks. For example, Aristotle used philosophy to deduce that heavy objects fall faster than light objects, but Galileo conducted an experiment to prove that Aristotle was wrong. He was even courageous enough to say that Aristotle, then the most acclaimed intellect in history,‘wrote the opposite of truth’.

      When Kepler first heard about Galileo’s use of the telescope to explore the heavens, he probably assumed that Galileo had invented the telescope. Indeed, many people today make the same assumption. In fact, it was Hans Lippershey, a Flemish spectacle-maker, who patented the telescope in October 1608. Within a few months of Lippershey’s breakthrough, Galileo noted that ‘a rumour came to our ears that a spyglass had been made by a certain Dutchman’, and he immediately set about building his own telescopes.

      Galileo’s great accomplishment was to transform Lippershey’s rudimentary design into a truly remarkable instrument. In August 1609, Galileo presented the Doge of Venice with what was then the most powerful telescope in the world. Together they climbed St Mark’s bell-tower, set up the telescope and surveyed the lagoon. A week later, in a letter to his brother-in-law, Galileo was able to report that the telescope performed ‘to the infinite amazement of all’. Rival instruments had a magnification of about × 10, but Galileo had a better understanding of the optics of the telescope and was able to achieve a magnification of × 60. Not only did the telescope give the Venetians an advantage in warfare, because they could see the enemy before the enemy saw them, but it also enabled the shrewder merchants to spot a distant ship arriving with a new cargo of spices or cloth, which meant that they could sell off their current stock before market prices plummeted.

      Galileo profited from his commercialisation of the telescope, but he realised that it also had a scientific value. When he pointed his telescope at the night sky, it enabled him to see farther, clearer and deeper into space than anyone ever before. When Herr Wackher told Kepler about Galileo’s telescope, the fellow astronomer immediately recognised its potential and wrote a eulogy: ‘O telescope, instrument of much knowledge, more precious than any sceptre! Is not he who holds thee in his hand made king and lord of the works of God?’ Galileo would become that king and lord.

      First, Galileo studied the Moon and showed it to be ‘full of vast protuberances, deep chasms and sinuosities’, which was in direct contradiction to the Ptolemaic view that the heavenly bodies were flawless spheres. The imperfection of the heavens was later reinforced when Galileo pointed his telescope at the Sun and noticed blotches and blemishes, namely sunspots, which we now know to be cooler patches on the Sun’s surface up to 100,000 km across.

      Figure 15 Galileo’s drawings of the Moon.

      Then, during January 1610, Galileo made an even more momentous observation when he spotted what he initially thought were four stars loitering in the vicinity of Jupiter. Soon it became apparent that the objects were not stars, because they moved around Jupiter, which meant that they were Jovian moons. Never before had anybody seen a moon other than our own. Ptolemy had argued that the Earth was the centre of the universe, but here was indisputable evidence that not everything orbited the Earth.

      Galileo, who was in correspondence with Kepler, was fully aware of the latest Keplerian version of the Copernican model, and he realised that his discovery of Jupiter’s moons was providing further support for the Sun-centred model of the universe. He had no doubt that Copernicus and Kepler were right, yet he continued to search for evidence in favour of this model in the hope of converting the establishment, which still clung to the traditional view of an Earth-centred universe. The only way to break the impasse would be to find a clear-cut prediction that differentiated between the two competing models. If such a prediction could be tested it would confirm one model and refute the other. Good science develops theories that are testable, and it is through testing that science progresses.

      In fact, Copernicus had made just such a prediction, one which had been waiting to be tested as soon as the tools were available to make the appropriate observations. In De revolutionibus, he had stated that Mercury and Venus should exhibit a series of phases (e.g. full Venus, half Venus, crescent Venus) similar to the phases of the Moon, and the exact pattern of phases would depend on whether the Earth orbited the Sun, or vice versa. In the fifteenth century nobody could check the pattern of phases because the telescope had yet to be invented, but Copernicus was confident that it was just a matter of time before he would be proved correct: ‘If the sense of sight could ever be made sufficiently powerful, we could see phases in Mercury and Venus.’

      Figure 16 Galileo’s sketches of the changing positions of Jupiter’s moons. The circles represent Jupiter, and the several dots either side show the changing positions of the moons. Each row represents one observation taken on a particular date and time, with one or more observations per night.

      Leaving aside Mercury and concentrating on Venus, the significance of the phases is apparent in Figure 17. Venus always has one face illuminated by the Sun, but from our vantage point on the Earth this face is not always towards us, so we see Venus go through a series of phases. In Ptolemy’s Earth-centred model, the sequence of phases is determined by Venus’s path around the Earth, and its slavish obedience to its epicycle. However, in the Sun-centred model, the sequence of phases is different because it is determined by Venus’s path around the Sun without any epicycle. If somebody could identify the actual sequence of Venus’s waxing and waning, then it would prove beyond all reasonable doubt which model was correct.

      In the autumn of 1610, Galileo became the first person ever to witness and chart the phases of Venus. As he expected, his observations perfectly fitted the predictions of the Sun-centred model, and provided further ammunition to support the Copernican

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