Observing the Solar System

Wandering Stars

• Ancient Greeks noticed that the stars seemed to move BUT they stayed together in formations (the constellations.)
• They noticed that 5 stars "wandered" across the sky called them "Planets" greek word for wanderer

Early Greek Model of the sky

• Universe was perfect and unchanging

• Thought that the Earth was in the center of a rotating dome
  
• Earth was in the center and EVERYTHING revolved around the Earth this is the GEOCENTRIC MODEL
  
• PTOLEMY 140 A.D. said that planets move on little circles as they moved on bigger circles around the Earth in perfect circles-
• Started the idea that planets are in orbits
  
• His view lasted for 1400
  

Copernicus

• Early 1500's Polish astronomer Nicolaus Copernicus thought the Sun was in the center of the universe
• HELIOCENTRIC MODEL the planets revolve around the sun (based on Hipparchus about 170 BC)
• Still thought the planets moved in perfect circles but around the sun NOT the Earth

Galileo

• Italian astronomer/inventor lived in the late 1500's - early 1600's
• Late 1500's early 1600's (1609 telescope changed things drastically) Most people still believe Ptolemy's geocentric view of the universe.
• Galileo made two major observations with his telescope:
  • Observed the moon of Jupiter- showed that some object orbit something OTHER than the Earth, so why couldn't the Earth orbit something else?
  • Observed the phases of Venus. IF Venus orbited around the Earth, there is no way that we would see phases. THE ONLY WAY we would see phases of Venus is if both planets (Venus AND Earth) orbited the Sun
• He still thought they orbited in perfect circles

Brahe

• Danish astronomer late 1500's
• Late 1500's made very accurate observation of the movements of the planets, believed Copernicus wanted to prove Heliocentric Model

Kepler

• German mathematician Worked for Brahe in the late 1500's, later discovered the shape of orbits
• Used Brahe's observations and tried to calculate when planets would arrive in different parts of the sky, realized that if orbits were circles the calculations were wrong BUT IF the orbits were ELLIPTICAL then the calculations were always correct!
• He proved the shape of orbits! Came up with 3 laws that explain the orbital motion of ANYTHING
  1. All orbits are Ellipses
  2. As a planet gets closer to the sun it speeds up, when it is farther away it slows down
  3. There is a relationship between the distance an object is from the sun and the time it takes to orbit.
• He could explain the shapes of orbits but did not know why they had that shape

Newton