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# Geocentric vs Heliocentric Model ## Equinox Equinox represents the dates the Sun crosses the equator and when day and night are approximately equal length. This happens on March 21st and September 21st. ## Geocentric Model * The incorrect theory that the Sun and planets orbit around Earth. *...

# Geocentric vs Heliocentric Model ## Equinox Equinox represents the dates the Sun crosses the equator and when day and night are approximately equal length. This happens on March 21st and September 21st. ## Geocentric Model * The incorrect theory that the Sun and planets orbit around Earth. * Since the sun, moon, and the stars seemed to rotate around the Earth, people believed that Earth was the center of the universe. * Proposed by Aristotle and Ptolemy, it was the main theory for celestial motions for almost 2000 years. * Ptolemy gave planets another level of circular motion called epicycles to account for the non-circular orbits around Earth. ## Heliocentric Model * The correct theory that Earth and other planets in our solar system revolve around the Sun. * Copernicus in the 1500s proposed that the Earth revolved around the Sun. * This explained the retrograde motion where planets (Mars, Jupiter, and Saturn) seemed to reverse the direction of their orbit. Earth is on a smaller orbit so it can "lap" the other planets causing retrograde motion of other planets. ## Johannes Kepler Johannes Kepler, a German mathematician, calculated the movements of the planets and found that they moved in an ellipse (oval) rather than a circular orbit. His calculations explained the movement of the Sun, the Moon and the other planets through the sky. ## The Law of Universal Gravitation The Law of Universal Gravitation from Newton provided an explanation for the elliptical orbit of planets. The orbit is a result of the attractive force of gravity, balancing the tendency of a planet to move in a straight-line motion.