Galileo used mathematical calculation as well as observation of nature and was the first astronomer to use a telescope. With an instrument of his own 1._____ Galileo observed Jupiter and four of 2.______ moons, the phases of Venus and the spots on the sun. His 3._______ and calculations confirmed that Copernicus and Kepler were right. He saw 4._______ his own eyes and made other people 5._______ too that the earth was not the fixed center of 6._______ universe as Ptolemy had said. Galileo 7._______ made some important discoveries in mechanics. He did not 8.________ drop cannon balls from the Leaning Tower of Pisa 9._______ that all bodies fall at the 10._______ speed, but he did roll balls 11.________ a slope to show that the 12.__________ a body falls is proportionate 13._______ the square of the time it takes to fall. Galileo also noticed the regular 14._______ of the lamps in Pisa Cathedral; 15._______ gave him the idea of the pendulum, a device that enabled him to make the clock a scientific instrument for the first time.