Introduction

For fourteen hundred years Europeans had believed in the system of Ptolemy: the earth stands motionless at the centre of the universe, and the sun, moon and stars all revolve around it. Copernicus proposed instead that the sun is the centre; that the earth turns on its axis once a day and goes round the sun once a year; and that the 'retrograde' motion of the planets is merely an appearance seen from our moving earth. On the Revolutions has six books; the last five are full of mathematical calculation, while the first sets out in plain language the basic principles of the new system and answers the objections to the motion of the earth. The book set off the 'Copernican revolution', changed for ever humanity's place in the universe, and opened the age of modern science.