note that read Haec immatura a me iam frustra leguntur oy (‘These are at present too young to be read by me’). He later revealed that this was a coded anagram that when unravelled read Cynthiæ figuras æmulatur Mater Amorum (‘Cynthia’s figures are imitated by the Mother of Love’). Cynthia was a reference to the Moon, whose phases were already familiar, and Mother of Love was an allusion to Venus, whose phases Galileo had discovered.
The case for a Sun-centred universe was becoming stronger with each new discovery. Table 2 (pp. 34—5) compared the Earth- and Sun-centred models based on pre-Copernican observations, showing why the Earth-centred model made more sense in the Middle Ages. Table 3 (overleaf) shows how Galileo’s observations made the Sun-centred model more compelling. The remaining weaknesses in the Sun-centred model would be removed later, once scientists had achieved a proper understanding of gravity and were able to appreciate why we do not sense the Earth’s motion around the Sun. And although the Sun-centred model did not chime with common sense, one of the criteria in the table, this was not really a weakness because common sense has little to do with science, as discussed earlier.
Figure 17 Galileo’s precise observations of the phases of Venus proved that Copernicus was right, and Ptolemy wrong. In the Sun-centred model of the universe, shown in diagram (a), both the Earth and Venus orbit the Sun. Although Venus is always half-lit by the Sun, from the Earth’s point of view it appears to go through a cycle of phases, turning from a crescent to a disc. The phase is shown next to each position of Venus.
In the Earth-centred model of the universe, both the Sun and Venus orbit the Earth, and in addition Venus moves round its own epicycle. The phases depend on where Venus is on its orbit and on its epicycle. In diagram (b), Venus’s orbit is such that it is roughly between the Earth and the Sun, which gives rise to the set of phases shown. By identifying the actual series of phases, Galileo could identify which model was correct.
At this point in history, every astronomer should have switched allegiance to the Sun-centred model, but no such major shift took place. Most astronomers had spent their entire lives convinced that the universe revolved around a static Earth, and they were unable to make the intellectual or emotional leap to a Sun-centred universe. When the astronomer Francesco Sizi heard about Galileo’s observation of Jupiter’s moons, which seemed to suggest that the Earth was not the hub of everything, he came up with a bizarre counter-argument: ‘The moons are invisible to the naked eye and therefore can have no influence on the Earth and therefore would be useless and therefore do not exist.’ The philosopher Giulio Libri took a similarly illogical stance and even refused to look through a telescope on a point of principle. When Libri died, Galileo suggested that he might at last see the sunspots, the moons of Jupiter and the phases of Venus on his way to heaven.
The Catholic Church was similarly unwilling to abandon its doctrine that the Earth was fixed at the centre of the universe, even when Jesuit mathematicians confirmed the superior accuracy of the new Sun-centred model. Thereafter, theologians conceded that the Sun-centred model was able to make excellent predictions of planetary orbits, but at the same time they still refused to accept that it was a valid representation of reality. In other words, the Vatican viewed the Sun-centred model in the same way that we regard this sentence: ‘How I need a drink, alcoholic of course, after the heavy lectures involving quantum mechanics.’ This phrase is a mnemonic for the number π. By noting the number of letters in each word of the sentence, we obtain 3.141 592 653 589 79, which is the true value of π to fourteen decimal places. The sentence is indeed a highly accurate device for representing the value of π, but at the same time we know that π has nothing to do with alcohol. The Church maintained that the Sun-centred model of the universe had a similar status – accurate and useful, but not reality.
Table 3
This table lists ten important criteria against which the Earth-centred and Sun-centred models could be judged based on what was known in 1610, after Galileo’s observations. The ticks and crosses give crude indications of how well each model fared in relation to each criterion, and a question mark
| Criterion | Earth-centred model | Success |
|---|---|---|
| 1. Common sense | It seems obvious that everything revolves around the Earth |
 |
| 2. Awareness of motion | We do not detect any motion, therefore the Earth cannot be moving |
|
| 3. Falling to the ground | The centrality of the Earth explains why objects appear to fall downwards, i.e. they are being attracted to the centre of the universe |
|
| 4. Stellar parallax | There is no detection of stellar parallax, absence of which is compatible with a static Earth and a stationary observer |
|
| 5. Predicting planetary orbits | Very close agreement |
|
| 6. Retrograde paths of planets | Explained with epicycles and deferents |
|
| 7. Simplicity | Very complicated – epicycles, deferents, equants and eccentrics for each planet |
|
| 8. Phases of Venus | Fails to predict the observed phases |
 |
| 9. Blemishes on Sun and Moon | Problematic – this model emerges from an Aristotelian view, which also claims that the heavens are perfect |
|
| 10. Moons of Jupiter | Problematic – everything is supposed to orbit the Earth! |
|
indicates a lack of data. Compared to the assessment based on the evidence available before Copernicus (Table 2, pp. 34—5), the Sun-centred model now seems more convincing. This is partly down to new observations (points 8, 9 and 10) that were possible only with the advent of the telescope.
| Criterion |
Sun-centred model
|
|---|