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But Galileo had uses in mind for his ‘perspicillum’ other than spotting ships, providing curious occasional glimpses of the Moon and stars, and securing university tenure at Padua. He set about making systematic astronomical observations, recording them, and using his fine mathematical skills to draw conclusions about what they meant.

In the autumn of 1609, Galileo turned one of his new instruments on the Moon. Although the ancient Greeks had described the Moon as ‘earthy, with mountains and valleys’, conventional wisdom in Galileo’s day had it that it was perfectly smooth and spherical. Both Greek and Hellenistic astronomers and medieval scholars understood that the Moon shines by reflected sunlight, not by its own light. Through his telescope Galileo watched sunrise on the Moon’s surface and saw isolated bright dots in the dark portion expand and join with one another. Reminded of what he had observed when sunrise strikes mountain peaks on Earth, he speculated that the separate bright spots must be peaks and ridges, lit first by the Sun’s rays before these could penetrate to the lower areas of the Moon’s surface. It occurred to him that by studying the shadows of these features he might measure the heights of the peaks and ridges. Galileo arrived at an estimated height of four to five miles. Modern measurement of the particular range of lunar mountains that he studied has them no higher than 18,000 feet. Never mind that discrepancy. The more significant point was that the Moon was not, as many had supposed, smooth.

Aiming his telescope at things more distant than the Moon, Galileo began to make further discoveries. In January of 1610, using an instrument whose lenses he had ground himself with great care, he discovered three pinpricks of light near Jupiter, neatly lined up with the planet. Galileo watched, mystified and then increasingly excited, as the little stars and Jupiter
exchanged
positions in their line-up and varied in brightness over the course of several nights.
See Figure 3.7.

Galileo concluded that this remarkable heavenly quadrille ‘ought to be observed henceforward with more attention and precision’. Before long he found that there are four rather than three stars; that the stars move within a narrow range, always in line with Jupiter and with one another; that they stay with the planet when its motion becomes retrograde; that when they are furthest from Jupiter they are never closely packed together, but when they are nearer to Jupiter they are sometimes closely packed. The implication of this last was that if the stars are circling Jupiter, the orbits in which they move are not all the same. If the stars were following one another in the same track, it’s likely they would sometimes line up so as to seem (from our vantage point) to cluster when they are furthest from the star.

Galileo reasoned that these could only be satellites, ‘planets never seen from the beginning of the world up to our own time’, orbiting Jupiter in the same way the Moon orbits the Earth. The deeper importance of what he had discovered also didn’t escape him. Never again would it be possible to suppose that there was only one body that was the centre of all motion in the universe.

Galileo, being Galileo, soon found a way to capitalize on his discovery. He rushed into print with a book called
Sidereus Nuncius
, translated
Starry Message
(the quotations in
Figure 3.7
come from that book), calling on all astronomers to equip themselves with good instruments and turn them on Jupiter. He dedicated his book not to just any local nobleman, but to the powerful Grand Duke Cosimo II de’ Medici, of Tuscany, who had once been his pupil. He decided to name his discovery the Cosmican Stars – in honour of Cosimo – but soon thought better of that. It would sound too much like ‘cosmic’ and the significance of the name would be missed. He settled on the Medicean Stars. There were, after all, four stars and four Medici brothers.

Figure 3.7

(The large disc in this picture is Jupiter. Galileo assumed the three little ‘stars’ were part of the background of fixed, distant stars – though they made him ‘somewhat wonder’.)

(Jupiter seemed to have passed up the three stars, and Galileo ‘became afraid lest the planet might have moved differently from the calculation of astronomers’.)

(Galileo waited ‘with the most intense longing’.)

(Galileo decided the third star must be hidden by the planet and it also occurred to him, ‘changing from doubt to surprise – that the interchange of position belonged not to Jupiter but to the stars’.)

(Galileo noticed that one of the two visible stars was larger than it had been before and quite a bit larger than the other. He ‘decided unhesitatingly, that there are three stars in the heavens moving around Jupiter’.)

Galileo meanwhile hadn’t been neglecting other stars and planets and had found that, though his instrument transformed the planets into discs, the stars still looked like points of light. Furthermore there were astounding numbers of them that had never been seen before. The Milky Way, he discovered, is ‘nothing else but a collection of innumerable stars . . . many of them are tolerably large and extremely bright, but the number of smaller ones is quite beyond determination’. There was far, far more to the universe than anyone on the face of the Earth had ever supposed. Galileo hastily added some pages in the middle of his book to report these discoveries.

A copy of
Sidereus Nuncius
reached Kepler in Prague. He also heard about the discovery of Jupiter’s satellites through his friend Wackher von Wackenfels. Galileo asked Kepler for his opinion and the reply came in the form of a long letter that was later published as
Conversation with the Starry Messenger
. In it, Kepler discussed Galileo’s discoveries and theories and expressed his agreement. Galileo wrote back, ‘I thank you because you were the first one, and practically the only one, to have complete faith in my assertions.’ Galileo did not, however, respond to Kepler’s rather broad hint that he would enjoy owning one of Galileo’s telescopes, though he was sending them as gifts to many influential people. Galileo actually understood the principles of the telescope no better than Kepler, perhaps not as well, though Kepler never built one. When Kepler was working with Tycho’s data he had to study optics to learn how to eliminate errors due to the smearing (‘refraction’) of light as it passes through the Earth’s atmosphere. Kepler did, for a short while, have one of Galileo’s telescopes on loan from a mutual acquaintance.

Galileo’s self-marketing scheme was successful. His book appeared in March 1610 – quick publishing indeed – and by late summer he had accepted the Grand Duke’s offer of a job and moved back to Florence. He was now a celebrated and well-placed scientist and astronomer.

About the time Galileo must have been unpacking his equipment in Florence, the planet Venus came into a good position for viewing in the evening sky. Galileo examined the planet and the area around it, searching in vain for companions like the ones he had found around Jupiter. In a letter in mid-November to Cosimo’s brother Giuliano, ambassador in Prague, he wrote that there seemed to be no satellites around any of the planets except Jupiter. However, his study of Venus was to yield other significant results. Although most scholars give the credit to Galileo, there is some question whether it was he or his former student Benedetto Castelli who at this juncture remembered a suggestion that Copernicus had made in
De revolutionibus
, that Venus might supply important evidence in the case against an Earth-centred universe. It was certainly Galileo who proceeded to find the evidence.

When we visited the amusement park in
Chapter 2
and studied the moving lights, there was one possibility we failed to consider. Imagine once again the entire park plunged into darkness, with glowing lights attached to the heads of only a few of the riders. As we try to figure out what carnival rides might produce this pattern of movement – and what the park would look like in daylight – we should bear in mind that some of the pinpoints of light we see might not be light sources at all, but instead be shining by reflected light. Perhaps a bauble that is not a light source itself, on the head of one of the carousel horses, is reflecting the light cast from a nearby horseman. How would we know the difference?

Do any of the lights have ‘phases’ like the Moon? Do any of them sometimes appear as a distorted disc, a half, or a crescent? If we find that, might it indicate that, like the Moon, this is not a light source but a reflection of light coming from elsewhere? If so, perhaps we could use its ‘waxing and waning’ as a clue to its position and motion, and to the position and motion of the source of its light.

It was this line of reasoning that Galileo used in 1610, when
he
studied the planet Venus through his telescope. In Ptolemaic astronomy, Venus always lay between the Earth and the Sun. For that reason,
if
Venus sheds no light of its own but only shines with reflected sunlight, observers on the Earth should never see the face of Venus anywhere near fully lit. In other words, it should never be the near equivalent of a full Moon.
See Figure 3.8.

Figure 3.8