city which was uniform with regard to the centre of a third circle, the equant,[1] was only a most ingenious mathematical representation of the phenomena—a working hypothesis; it did not pretend to give a physically true description of the actual state of things in the universe.[2] No doubt there were many smaller minds to which this did not become clear, but both by the great mathematician who completed it, and by astronomers of succeeding ages the Ptolemean system was merely considered a mathematical means of computing the positions of the planets.
When astronomy towards the end of the fifteenth century again began to be cultivated in Europe, the inconvenience of the extremely complicated system became felt, and soon the great astronomer of Frauenburg conceived how a different system might be devised on the basis of the earth's motion round the sun. But Copernicus did a great deal more than merely suggest that the earth went round the sun. He worked out the idea into a perfect system, and developed the geometrical theory for each planet so as to make it possible to construct new tables for their motion. And though he had but few and poor instruments, and did not observe systematically, he took from 1497 to 1529 occasional observations in order to get materials for finding the variations of the elements of the orbits since the time
- ↑ The earth, the centre of the deferent, and the centre of the equant were in a straight line and equidistant; only in the case of Mercury the centre of the equant was midway between the earth and the centre of the deferent.
- ↑ Perhaps we may illustrate this by an example from modern science. When the deflection of a magnetic needle in the neighbourhood of an electric current was first discovered, some difficulty was felt in giving a rule for the direction in which either pole of a needle is deflected by a current, whatever their relative positions may be, until Ampere suggested that if we imagine a human figure lying in the current facing the needle, so that the current comes in at his feet and out at his head, then the deflection of the north-seeking pole will be to his left. Nobody ever suspected Ampere of believing that there really was a little man lying in the current, but to many people in the Middle Ages the epicycles were doubtless really existing.
