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and the instant that it was totally hidden. He then found that that length of time was the same as the length of time during which the Moon was totally hidden. He reasoned that the breadth of the Earth’s shadow where it was crossed by the Moon must therefore be approximately twice the diameter of the Moon itself ( Figure 1.4c ). If, as he thought, the angle formed at the point of the Earth’s shadow was the same as the angular size of the Moon, that gave him only one distance at which to put the Moon where it would cover half of the area of the shadow.
    Aristarchus concluded that the Moon was ¼ the size of the Earth, and that the distance to the Moon was about 60 times the radius of the Earth. Both of those values are close to the modern values. Using Eratosthenes’s calculation of the Earth’s radius, Aristarchus arrived at an actual distance to the Moon in stades. He had less success with the distance to the Sun. His earlier estimate – that the Sun’s distance is about 19 times the Moon’s distance – was in error, and a second approach he tried, though it was ingenious and correct, required timing the phases of the Moon with a precision impossible in his day.
    It was another of Aristarchus’s ideas that secured his place much more firmly in the annals of astronomy. Hearing of it, one has a chilling sensation of stumbling into a prophetic vision. For Aristarchus suggested, 17 centuries before Copernicus, that the Earth is not the unmoving centre of everything but instead moves round the Sun, and that the universe is many times larger than anyone in his time thought – perhaps infinitely large.
    For centuries it had been widely assumed that the Earth was the centre of everything. The accepted picture of the cosmos was a series of concentric spheres – spheres embedded one within the other – with the Earth resting motionless at the centre of the system. ( See Figure 1.5. )
    Plato and Euxodus of Cnidus, a younger contemporary of Plato, had introduced this model, and Aristotle’s model of the universe was a further development of it, though he differed from Euxodus as to the number and nature of the spheres. However, it wouldn’t be correct to think that everyone, without exception , since the dawn of human thought had agreed that the Earth was the centre and didn’t move. Some Pythagorean thinkers had decided in the fifth century BC , largely for symbolic and religious reasons, that the Earth was a planet and that the centre of the universe must be an invisible fire. Heraclides of Pontus, a member of Plato’s Academy under Plato, proposed that the daily rising and setting of all the celestial bodies could be nicely explained if the Earth rotated on its axis once every 24 hours.
    Figure 1.5

    But Aristarchus went further. Although information about his theory of a Sun-centred cosmos comes second-hand, no one disputes his authorship of the idea because there is plenty of secondary evidence. According to Archimedes:
    Aristarchus of Samos brought out a book of certain hypotheses, in which it follows from what is assumed that the universe is many times greater than that now so called. He hypothesizes that the fixed stars and the Sun remain unmoved; that the Earth is borne round the Sun on the circumference of a circle . . . and that the sphere of the fixed stars, situated about [that is, centred on] the same centre as the Sun, is so great that the circle in which he hypothesizes that the Earth revolves bears such a proportion to the distance of the fixed stars as the centre of the sphere does to its surface.
    Aristarchus had done no less than move the centre of the cosmos to the Sun. In this astounding turn-about, the Earth moves round the Sun and, rather than the sphere of the fixed stars making a revolution of the heavens once every 24 hours, it is the Earth that turns, rotating on its axis – as Heraclides had suggested. The stars are extremely far away. The implication is, infinitely far.
    Did Aristarchus