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"Moving Copernican System" In the early 16th century, Copernicus, who was born in Poland, proposed a heliocentric theory in which the Earth revolved around the sun. Now we know that this theory was correct. In the early 17th century, Kepler showed, based on the observations, that the orbits of planets did not form perfect circles, but rather traveled elliptical orbits, with the sun at a focus point. Galileo also advocated the heliocentric theory based on his astronomical observations. Finally, Newton discovered the Law of Universal Gravitation toward the end of 17th century, proving the correctness of the Copernican System. "Moving Ptolemaic System" More than 1,800 years ago, the Greek astronomer Ptolemy from Alexandria proposed a cosmic structure called the Ptolemaic System in which Earth was stationary at the center of the cosmos, while the moon, the sun, planets, and stars revolved around it. This theory held sway for over 1,300 years but was eventually discredited due to various observations.