Galileo Galilei (1564-1642) contributed to what we know about the solar system in several significant ways: He was the first to use a telescope to systematically observe the heavens. His observations about Earth’s sun, Moon, and stars of the Milky Way lent further credibility to the Copernican view that Planet Earth was not the center of the universe. Unfortunately, his contributions and views almost led to his being executed for heresy.
♦ Galileo’s improvements of the telescope
Galileo did not invent the telescope he used to observe the skies, but he did improve on its principle and turned the ordinary spyglass into his own instrument that could magnify objects 20 times. He was the first to use this remarkable improvement to systematically study the heavens, and it is likely that his gazing through the telescope at the sun caused his eventual blindness.
♦ Galileo’s discoveries and observations
In 1610, Galileo published a short book called the “Starry Messenger.” Published in Venice, his book became an immediate sensation. Galileo observed that the Moon was not a smooth, round body, but had mountains. He claimed to have proved that the Milky Way consisted of individual stars and that he saw four small bodies orbiting Jupiter (known now as the Galilean Moons of Jupiter).
In his 1612 “Discourse on floating bodies,” Galileo described the revolutionary notion that the Sun had to be rotating on its axis because the dark sunspots were in motion. Conventional scientific and religious wisdom of the day was that the Sun was a perfect and unchanging heavenly body. If the Sun rotates, Galileo reasoned, it also seemed likely that the Earth and other planets in the solar system rotated.
◊ Jupiter’s moons
Galileo’s observations of Jupiter and the four spots of light he patiently tracked around that planet supported the arguments made by Copernicus. Again, conventional wisdom tried to disprove Copernicus with this argument: if moons orbited planets and planets orbited the Sun, planets would have to leave their moons behind as they orbited the Sun; therefore, the Earth must be fixed in place. Galileo’s observations showed the opposite: planets like Earth and Jupiter actually did have orbiting moons, which kept their orbit as the planets themselves circled the Sun. Conventional wisdom at the time relied on complicated, specious (and somewhat ridiculous) explanations on the positioning of planets and their satellites and their relative position to the Earth. Galileo’s observations simplified the matter through simple observation and empirical reasoning.
◊ Phases of Venus
Using his telescope, Galileo observed that Venus cycled through phases like our Moon. Probably the most important observation in human history, this discovery was proof positive that Copernicus was correct. The notion was stunningly simple: Venus is never far from the Sun in Earth’s sky. If the old Ptolemaic system were correct, Venus should always be in the same phase (which it was not), because of a stationary Earth. What Galileo saw were various views (phases) of Venus over time with aspects that can only be explained by its orbit around the Sun relative to a rotating Earth.
◊ The “ears” of Saturn
Galileo was the first to view Saturn through a telescope and to see what he called “ears.” He noted that that they would disappear and reappear from time to time. These “ears” were actually Saturn’s rings and their appearance and disappearance were a result of Saturn’s orbital aspect from the Earth as Saturn completed its orbit around the Sun. It would take an improved telescope and a 1655 observation by Christian Huygens to verify that the “ears” were Saturn’s rings.
♦ Galileo ran afoul of Religious authorities
So Galileo was indeed a pivotal force in the science of astronomy. He was, unfortunately, up against the state religion of the Catholic Church, which viewed the universe as a perfect creation of God with the Earth at its center. To purport otherwise was dangerous religious heresy and it brought Galileo to the attention and later persecution of the Holy Inquisition. In the end, Galileo recanted everything to save his life. He escaped being burnt at the stake and spent the final years of his life under house arrest. That “end,” however, was not the final chapter. In 1992, Pope John Paul II acknowledged that the Church had done Galileo wrong by blending theology too closely with scientific discovery.