When Galileo Galilei began studying the evening sky with his telescopes, astronomy took a huge leap forward.
Though Galileo did not invent the telescope, he was the first to use it systematically to observe celestial objects and record his discoveries. His book, Sidereus Nuncius, or The Starry Messenger, was first published in 1610, which made him famous. In it he reported his observations of the moon, Jupiter, Venus, the sun and objects in the Milky Way. These observations and his interpretations of them eventually led to the demise of the geocentric, Earth-centered, model of the universe and the adoption of the heliocentric, sun-centered, model as proposed in 1543 by Nicholas Copernicus.
The instruments Galileo used were a type of home-made refracting telescope, which had a convex objective lens mounted at one end of a long tube with a concave eyepiece mounted at the other end. Though his initial version only magnified eight times, he soon created a telescope having a power of 20 times, which he used for his observations in the Sidereus Nuncius. The main problem with his instruments was their very narrow field of view, about half the width of the moon. Typically, one of Galileo’s telescopes had a focal length of 52-inches with a one inch aperture, which magnified 14 times. Through his systematic use and refinement of these telescopes, Galileo would make several unique discoveries that would change the world and the way people see the universe.
According to Aristotle, the moon was in the heavens above the sublunary sphere (the region of the geocentric cosmos below the moon, consisting of the four classical elements: earth, water, air, and fire) and hence should be perfect. However, Galileo observed the surface of the moon not to be smooth, even and perfectly spherical, but to the contrary, to be uneven, rough and crowded with craters and bulges. It was like the face of the Earth, which is marked with chains of mountains and depths of valleys. He was able to calculate the heights of these mountains by measuring the lengths of their shadows cast by the sun and applying geometry.
Observations of the planet Jupiter over successive evenings revealed four star-like objects in a line with it, which moved from night to night, sometimes disappearing behind, or in front of the planet. Galileo correctly inferred that these objects were moons of Jupiter that orbited it, just as our moon orbits Earth. For the first time, objects had been observed orbiting another planet, thus weakening the hold of the Ptolemaic geocentric model. Today these four moons are known as the Galilean satellites; Io, Europa, Ganymede and Callisto.
Venus was also observed to go through a sequence of phases similar to the moon. This could not be explained in the Ptolemaic model but could be accounted for by the sun-centered Copernican model that had the other planets orbiting the sun. Galileo used this discovery to help consolidate his support of the Copernican model.
Along with contemporaries such as Thomas Harriot, David Frabicius and Christoph Scheiner, Galileo observed dark splotches that appeared to move across the surface of the sun. Debate centered on whether these were satellites of the sun or actual spots on its surface. Galileo, in his 'Letters on Sunspots' supported the spot interpretation and used it to show that the sun was rotating. Its blemishes and imperfections again undermined the Aristotelian ideal of a perfect cosmos.
When observing the planet Saturn, Galileo noted two appendages”at the sides of the planet which disappeared then later reappeared. Though unsure of what they were, it was also an inconsistency with the perfect geocentric model. It was not until 1656 that the Dutch scientist, Christiaan Huygens, using a much more advanced telescope correctly described them as rings.
Even through a telescope, the stars still appeared as points of light. Galileo suggested that this was because of their immense distance from Earth. This then eased the problem posed by the failure of astronomers to detect stellar parallax that was a consequence of Copernicus’ model. On turning his telescope to the band of the milky way, Galileo saw it resolved into thousands of hitherto unseen stars. This also posed the question as to why there were invisible objects in the night sky.
So within a couple years, Galileo had turned the conventional Ptolemaic Earth-centered world view up-side-down with his ideas and publications, but in doing so he had also made an enemy of the Catholic church. The Roman Inquisition tried Galileo in 1633 and found him vehemently suspect of heresy, sentencing him to indefinite house arrest until his death in 1642. He would not be vindicated by the Church until 1992.