The Copernican thesis was supported and developed by two distinguished astronomers at the beginning of the next century--Kepler (1571-1630) and Galileo (1564-1642), one a German, the other an Italian. Kepler taught astronomy for a number of years at Gratz and subsequently made his home in Prague, where he acquired a remarkable collection of instruments [Footnote: From Tycho Brahe, whose assistant he was in 1600-1601.] that enabled him to conduct numerous interesting experiments. While he entertained many fantastic and mystical theories of the "harmony of the spheres" and was not above casting horoscopes for the emperor and for Wallenstein, that soldier of fortune, [Footnote: See below, pp. 223, 226.] he nevertheless established several of the fundamental laws of modern astronomy, such as those governing the form and magnitude of the planetary orbits. It was Kepler who made clear that the planets revolve about the sun in elliptical rather than in strictly circular paths. Galileo popularized the Copernican theory. [Footnote: Another "popularizer" was Giordano Bruno (c. 1548-1600).] His charming lectures in the university of Padua, where he taught from 1592 to 1610, were so largely attended that a hall seating 2000 had to be provided. In 1609 he perfected a telescope, which, although hardly more powerful than a present-day opera glass, showed unmistakably that the sun was turning on its axis, that Jupiter was attended by revolving moons, and that the essential truth of the Copernican system was established. Unfortunately for Galileo, his enthusiastic desire to convert the pope immediately to his own ideas got him into trouble with the Roman Curia and brought upon him a prohibition from further writing. Galileo submitted like a loyal Catholic to the papal decree, but had he lived another hundred years, he would have rejoiced that almost all men of learning--popes included--had come to accept his own conclusions. Thus modern astronomy was suggested by Copernicus, developed by Kepler, and popularized by Galileo.