illuminative idea was engendered. Presently, however, there arose a wonderful star-gazer at Samos (where Pythagoras had been born almost three centuries before), who conceived a method of demonstrating, roughly at any rate, the true relations of sun, moon, and earth. This man was named Aristarchus. He worked in the early part of the third century b.c. By studying the apparent size of the moon's disc and noting the alterations in this, Aristarchus was led to make a singularly penetrating estimate of the moon's apparent size. As he conceived it, the moon is about one-thirtieth the size of our earth.
That was a good beginning. But how make any estimate of the actual size of the sun, since his disc seems never to vary to a measurable extent? The genius of Aristarchus was equal to this task. He conceived a plan of measurement at once marvellous in its simplicity and demonstrative in its result. He reflected that since, as Anaxagoras had taught, the moon shines by light received from the sun, the moment when the moon as viewed from the earth is exactly at the half will be the moment when the line from the earth to the moon is at right angles to the line joining the moon and the sun. If then at this moment the angular distance between the sun and the moon is measured, that angle determines the precise shape of the right-angled triangle having the earth, the moon, and the sun at its respective angles. A simple geometrical drawing of this right-angled triangle will show, at a glance, the relative lengths of the two sides which represent the relative distances from the earth to the moon and to the sun. Aristarchus made such a measurement. Unfortunately his instruments lacked precision, and again it is not possible to make sure of the precise moment when the moon is at the half. So the result of the experiment of Aristarchus, till corrected by the modern astronomers, seems wide of the truth. Yet the measurements suffice to demonstrate the fact that the sun is many times as distant as the moon, and, that being true, his measurement of the apparent size of the sun's disc demonstrated to his satisfaction that the sun is immensely larger than the earth. If this be true, said Aristarchus, then it seems absurd to suppose that the gigantic sun can be a satellite of our tiny earth; rather must we reverse the terms and suppose that the sun, and not the earth, is the centre of our universe.
Here then is a clear conception of the mechanism of the solar system as we know it. Contemplating this astronomer of Samos, then, we are in the presence of a man who had solved in its essentials the problem of celestial mechanics. Had his teachings found vogue, the story of science would be very different from what it is. We should then have no tale to tell of a Copernicus coming upon the field fully 1700 years later with the revolutionary doctrine that our world is not the centre of the universe. We should not then have to tell of the persecution of a Bruno or of a Galileo for teaching that doctrine in the seventeenth century of an era which did not begin until two hundred years after the death of Aristarchus. But we are here concerned, not with what might have been, but with what was. We know that the teaching of the astronomer of Samos did not win its way. It was frowned upon by most contemporaries and perhaps by the immediate successors of Aristarchus, and soon it came to be ignored if not forgotten. But that detracts nothing from the merit of Aristarchus himself. His clear and unequivocal anticipation of the heliocentric doctrine, which we now associate with the name of Copernicus, barren though it was of immediate result, was surely one of the most remarkable of all the triumphs of scientific genius in any age.
The man of all others who might have given currency to the doctrine of Aristarchus, had he chosen to uphold it, was the great Rhodesian astronomer, Hipparchus, who lived about one hundred and fifty years later and who is remembered as the greatest observing astronomer of antiquity. But unfortunately Hipparchus could not accept the heliocentric idea. He knew of the work of Aristarchus, but to him the evidence seemed to uphold the geocentric theory. He was a man who spoke with such authority that his verdict alone, perhaps, would have sufficed to discredit the Aristarchian theory; yet Hipparchus had at command a means of demonstrating the
