 Look up at the night sky See the stars move across the sky The moon and the Sun do the same thing. They rise and set It's not surprising that ancient peoples viewed the earth as fixed and all celestial objects revolved around us The ancient Greeks such as Plato, Aristotle, and culminating in Ptolemy constructed a cosmology with the earth surrounded by a number of celestial spheres That rotated around the earth each day There was a sphere for the moon one for the Sun one for each planet and One for all the fixed stars Planets were identified as different from stars because they changed their position over time Whereas the stars were seen to be eternally fixed in place The Earth-centric model stood the test of time for over 1500 years It wasn't until the 16th century that things started to change When Nicholas Copernicus proposed to put the Sun at the center of the solar system In so doing he put the earth into rotational motion about an axis and he put the earth into revolutionary motion around the Sun But putting the earth into motion was hard to swallow for most people Copernicus's idea didn't really start to take hold until the early 17th century When considerable evidence for the Copernican model was compiled by the likes of Tycho Bra Johannes Kepler and Galileo Galilee Tycho Bra With mural quadrants Sextance and his naked eye Used parallax measurements to find distances to the planets He focused on Mars and tabulated volumes of data on a daily basis Using this information Kepler found that the orbits of the planets including the earth were ellipses And Galileo using the newly invented telescope discovered that the Milky Way cloud was actually stars That the Sun had spots on it that indicated that the Sun was rotating That Venus as phases like the moon indicating that it goes around the Sun and Jupiter as four moons Imagine how it must have felt When Galileo first saw these moons All the world believed that everything revolved around the earth and here you are looking at moons that are orbiting Jupiter Not the earth