Greeks

• Discovered the planets

• Planet is the greek word meaning wanderers

• Called wanerers because they sometimes moved in the opposite direction from the rest of the visible stars in the sky

• Geocentric - Earth is at the center of the solar system

• all planets, stars and our sun revolves around the Earth

Ptolemy’s model

• Greek astronomer

• Planets move on small circles along their orbit

• animation

Heliocentric

• Sun at the center of the solar system

• Copernicus 1473-1543

• Worked out the arangement of the known planets and how they moved around the sun

• (just a way to know where a planet is located at any given time)

• Animation

Galileo 1564-1642• Used telescope to see

moons revolving around Jupiter

• Why was this important?• Also noticed that full

phase of Venus is much smaller than cresent phase of Venus

• Why was this important?• Contradicts previous view

that everything in the solar system revolves around the Earth.

• Venus is further from the Earth at full phase. Therefore, it is smaller in size.

• When Venus is in cresent phase it is close to Earth. Larger.

Venus Phases from Earth

comparison between geocentric and heliocentric models

Tycho Brahe 1546-1601

• Over 20 years of detailed observations recording the location of the planets

Kepler 1571-1630

• Found that the orbit of each planet is an elipse

• Ellipse - Oval shaped, 2-foci

Ellipse vs. Circular Orbit