1. Early Astronomy

2. Geocentric Model – Geo means Earth
Greeks
Greeks saw 5 planets that would wander among the other stars. Why only 5?
No Telescopes
Geocentric Model – Geo means Earth
Earth is the center of the revolving planets and sun.
Ptolemy- thought that the planets moved in little circles that moved on larger circles that orbited Earth.
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3. The Heliocentric Model
Copernicus- came up with the model that all the planets revolved around the sun ( Helios means sun in Greek) because the early model did was inadequate a predicting the positions of the planets.
This model was not widely accepted because the planets and sun appeared to move around the Earth.
Galileo- confirmed the heliocentric model by discovering 4 satellites (moons) around Jupiter and that Venus had phases similar to the moon. This proved that not everything in the sky revolved around Earth.

4. Orbits Planets orbits were an elliptical shape like an oval
Kepler- discovered the elliptical orbits by using Brahe’s observations to match his calculations of Mar’s orbit.
Inertia- Planets or any objects will continue moving in a straight line until a force acts to stop a motion.
Why then on Earth do things stop?
Forces : Gravity, Friction
Newton- concluded that inertia and gravity from the sun combine to keep the planets in orbit.