Galileo
Unit 2 : Activity 2 : Ptolemaic               Model : Copernican Model : Brahe : Kepler : Galileo : Newton : Unit         Exam
Galileo
Galileo Galilei was the first astronomer to use a telescope              to study the heavens. Galileo made a number of observations that finally              helped convince people that the Sun-centered solar system model (the              heliocentric model), as proposed by Copernicus, was correct. These              arguments can be divided into two kinds: Those that proved that the              Ptolemaic model was incorrect; and those that undermined the broader              philosophy of Aristotelianism that included the Ptolemaic model. We'll              first consider some philosophically important observations and then              the ones that proved Venus, at least, goes around the Sun and not              around Earth.
Sun and Moon
One of the ideas that made Aristotelianism popular with the church              during the middle ages was that the heavens are perfect. This also              meant that they were unchanging, because if they change then either              they weren't perfect before or they won't be perfect after the change.
Galileo discovered spots on the Sun and also saw that the surface              of the Moon was rough. People really tried hard to account for these              observations without making the heavens imperfect; one suggestion              was that over the mountains of the Moon there was a layer of clear              crystal so the final surface would be smooth and perfect!
Jupiters Moons
Galileo saw near Jupiter what he first thought to be stars. When              he realized that the stars were actually going around Jupiter, it              negated a major argument of the Ptolemaic model. Not only did this              mean that the Earth could not be the only center of motion, but also              it knocked a hole in another argument. The supporters of the Ptolemaic              model argued that if the Earth were moving through space, the Moon              would be left behind. Galileos observations showed that the              moons of Jupiter were not being left behind as Jupiter moved.
Phases of Venus
One observation definitely disproved the Ptolemaic model, although              it didn't prove that Copernicus was right (as Tycho Brahe pointed              out). This was the observation that Venus has phases, much like our              Moon does.
To the naked eye, Venus always appears as a bright dot in the sky.              With a telescope, however, it is fairly easy to see the phases of              Venus. Just as the Moon has phases, Venus too has phases based on              the planets position relative to us and the Sun.
There was no way for the Ptolemaic model (Earth centered solar system)              to account for these phases. They can only occur as Galileo saw them              if Venus is circling the Sun, not the Earth.