PHYSICS 20A Lecture Notes - Lecture 6: Nicolaus Copernicus, Firmament, Heliocentrism
Physics 20A – Lecture 6 – Gravity & Orbits
Geocentric to Heliocentric
• Geocentric model – Ptolemy’s model of space puts earth at the center
o Everything revolves around earth
• Heliocentric model – Nicolaus Copernicus develops first modern sun-centered model of solar system
o Cons:
▪ Assumes planets move on perfect circles—epicycles
▪ Predicting planet motion with this model was about as faulty as geocentric predictions
o Pros:
▪ Can derive relative distances to planets and orbital periods of planets
▪ Explains the reason for the seasons
o Copernicus’s (outdated) Principles:
▪ Earth ≠ center of the universe
▪ Sun = center of the universe
▪ We cannot tell the distance between earth and the sun from the height of the firmament
− Firmament – distant stars
▪ Earth’s motion causes the motion we perceive in the firmament
▪ Earth’s motion causes the retrograde and direct motion we perceive of the planets
▪ Earth’s motion explains many apparent inequalities in space
Planetary motions
• Earth’s orbit around the sun = 1 year
• Mars’s orbit around the sun = 1.88y
• Retrograde motion
o Both Earth and Mars are moving
o At a certain point in their orbits, earth and mars will align
o From earth, as mars starts to align with the earth, it will appear to move backward in the sky
This causes retrograde motion
Mars’s path viewed from Earth
The Orbits
Blue – Earth
Red – Mars
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