N this book an attempt has been made to trace the history of man's conception of the Universe from the earliest historical ages to the completion of the Copernican system by Kepler in the seventeenth century. Among the various branches of physical science there is no other which in its historical development so closely re?ects the general progress of civilisation as the doctrine of the position of the earth in space and its relation to the planetary system. In this we may follow man's gradual emancipation from primitive ideas during the rise of Greek philosophy and science, his relapse into those ideas during the ages following the destruction of the seats of Greek culture, and the rapid advance of knowledge after the revival of learning at the end of the Middle Ages.