In 1592 he began lecturing on mathematics at the Univ. of Padua, where he remained for 18 years. There, in 1609, having heard reports of a simple magnifying instrument put together by a lens-grinder in Holland, he constructed the first known complete astronomical telescope. Exploring the heavens with his new aid, Galileo discovered that the moon, shining with reflected light, had an uneven, mountainous surface and that the Milky Way was made up of numerous separate stars. In 1610 he discovered the four largest satellites of Jupiter, the first satellites of a planet other than Earth to be detected. He observed and studied the oval shape of Saturn (the limitations of his telescope prevented the resolving of Saturn's rings), the phases of Venus, and the spots on the sun. His investigations confirmed his acceptance of the Copernican theory of the solar system; but he did not openly declare a doctrine so opposed to accepted beliefs until 1613, when he issued a work on sunspots. Meanwhile, in 1610, he had gone to Florence as philosopher and mathematician to Cosimo II de' Medici, grand duke of Tuscany, and as mathematician at the Univ. of Pisa.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.