PX282 - I1 - models

greeks

• the solar system appears geocentric (earth centered) except for occasional retrograde motion of planets (moving backwards on their paths)

image: stellarium

• this was explained by ptolemy with epicycles*, which is the circular motion around an average circular motion

image: university of viginia

image: encyclopedia brittanica, 1st ed., 1771

copernicus

• copernicus (1543) proposed the heliocentric model (sun centered),
• it is simpler, with a natural explanation of retrograde motion (as the earth overtakes other planets)
• this was highly controversial, and it still needed epicycles as it assumed circular orbits

kepler

• kepler developed three laws to fit precise observations by tycho brahe