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176 were developed. The physical objections to the earth's motion, which to us seem so easy to refute, were in the sixteenth century most serious difficulties, and the merits of Galileo in conceiving the principles of elementary mechanics and fixing them by experiments must not be underrated. Neither should the advantage be forgotten which the seventeenth century had over the sixteenth from the invention of the telescope, which revealed the shape of the planets, the satellites of Jupiter, and the phases of Venus, and thus placed the planets on an equal footing with the earth, to which the unassisted vision could never have seen any similarity in them.

Tycho Brahe evidently was not content with a mere geometrical representation of the planetary system, but wanted to know how the universe was actually constructed. He felt the "physical absurdity" of letting the earth move, but, on the other hand, the clearness of mind which made him so determined an opponent of the scholastic philosophy enabled him to see how unfounded some of the objections to the earth's motion were. In a letter to Rothmann in 1587 Tycho remarks that the apparent absurdity is not so great as that of the Ptolemean idea of letting a point move on one circle with a velocity which is uniform with regard to the centre of another circle. He adds that the objections which Buchanan had made to the revolution of the earth in his poem on the sphere are futile, since the sea and the air would revolve with the earth without any violent commotion being caused in them. But all the same he thought that a stone falling from a high tower ought to fall very far from the foot of the tower if the earth really turned on its axis. This remark is made in another letter to Rothmann in 1589, in which he made several objections to the annual motion of the earth. The immense space between Saturn