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the apparent motion of a relatively close object with respect to a more distant background as the location of the observer changes

Geocentric Model

earth stationary at the center of the universe

Heliocentric Model

sun stationary at the center of the universe


first to suggest the geocentric model


noticed that planets speed-up, slow-down, and even reverse direction of motion (retrograde motion). Model contained epicycles.


a large circle encircling Earth, on which an epicycle moves


each plant rides on a small _____ whose center in turn rides on a larger circle called the deferent

Supported the Geocentric Model

stars don't seem to show any parallax

Supported the Geocentric Model

we don't feel as though the Earth moves

Supported the Geocentric Model

objects fall toward Earth, not the sun


reintroduced the Heliocentric model

Copernican Model

model accounted for retrograde motion. Lacked uniform circular motion (same speed all along the circle)

Tycho Brahe

made own instruments to observe the universe. Believed that the sun and moon orbited the Earth and then all the other planets and stars orbited around the sun


said that orbits are ellipses not circles

Kepler's 1st Law

planetary orbits are ellipses, sun at one focus


closest to sun


farthest from sun

Kepler's 2nd Law

draw imaginary line from planet to sun. sweeps over equal area in equal time. Distance between the planet and the sun changes as the planet orbits the sun.

Kepler's 3rd Law

planet's orbital period squared is proportional to cube of its average distance from sun


confirmed Copernicans ideas with three major results from use of telescope

Galileo's Finding that Contradicts Ptolemy Model

phases of Venus: crescent when close to Earth, gibbous when furthest away. Concluded Venus revolves around the sun.


1st suggested heliocentric model

Known Planets

Mercury, Venus, Mars, Jupiter, Saturn (sun and moon)

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