Historical Models of our Solar System and Kepler’s Laws of Planetary Motion

Geocentric Model Earth is center of our Solar System Aristotle- Over 2000 years ago, Unexplained on how planets appear to move backwards Ptolemy- Planets move in small circles or epicycles

Heliocentric Model Sun is center of our

Galileo Supporter of Copernicus’s Heliocentric theory House arrest Observed moons orbiting Jupiter and theorized objects can revolve around other planets not just Earth

Solar System precisely measured Tycho Brahe- studied Solar System and made very accurate recordings of his observations Tycho’s assistant, Kepler, used information for the details of orbits

Kepler’s Laws of Planetary Motion 1 st Law - Law of Ellipses Each planet orbits the sun in a path called an ellipse or elongated circle Ellipse is a closed curve whose path is determined by 2 points or foci within the ellipse Focus 1 is the Sun and Focus 2 is an imaginary point Semi-major axis = average distance from sun (AU); ½ major axis

Eccentricity Degree of elongation or shape of planet’s orbit Ratio between the foci and length of major axis Circular =0 Very elongated =1 Shapes

2 nd Law – Law of Equal Areas An imaginary line between the Sun and a planet sweeps out equal areas in equal times as the planet travels around the ellipse.

3rd Law – Law of Periods Mathematical relationship P 2 = a 3 a= semi-major axis (planets average distance from Sun measured in AU’s) p= planet’s orbital period (time)

Newton Used Kepler’s Laws of planetary motion and published Principia for short. In this publication, considered to be the greatest piece of scientific literature ever written. Contains Newton’s laws of motion including universal gravitation.

Einstein ( ) Changed Newton’s gravitational theory based on findings of Mercury’s orbit. Developed Theory of Relativity. It completely changed the way we study gravity and even changed our understanding of the universe.