Chapter2 Review
Geocentric Model
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Heavens were made of perfect, rotating, crystalline spheres
Planets, Sun, Moon and Stars were carried on these perfect spheres
Spheres move in uniform circular motion, but each sphere moved at different speeds
Aristotle rejected the Heliocentric Model
o If the Earth spun on an axis, why didn't objects fly off the Earth?
o If the Earth was in motion around the sun, why didn't it leave behind the Birds in
the air?
o If the Earth was in orbit around the sun, why wasn't a parallax effect observed?
(see animation)
Problems with the Geocentric Model
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Planets do not move in uniform circular motion
Sometimes Planets move in retrograde motion (backwards from their normal motion)
Planets would vary in brightness (planets would increase in brightness during retrograde
loop)
Epicycle Solution
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Explained retrograde motion
Explained varying brightness of the planets
Not accurate enough to account for the motion of the planets
Cannot account for the large brightness variations of the planets
Critics of the Geocentric (Earth Centered) Model
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Not accurate enough to predict position of the planets far into the future
Planetary positions of the models need to be reset from time to time (similar to resetting a
clock)
Epicycles were getting too complicated
Epicycles are not aesthetically pleasing
Epicycles cannot account for the large brightness variations of the planets
Heliocentric
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the daily motion of the Sun, Planets and Stars are caused by the rotation of the Earth on
its axis
Earth orbits the Sun, so the Sun appears to move against the stars
Earth's faster orbital motion overtakes the slower outer planets, so they appear to move
backward
Mercury & Venus move in retrograde motion and found close to the sun because they are
interior planets
explains retrograde motion and the varying brightness of the planets
Problems with the Copernican Heliocentric Model
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Did not predict the position of the planets any more accurately than the Geocentric model
Copernicus still assumed that the planets move in circular orbits with constant speed
Copernicus also had to resort to epicycles to make predictions of planetary positions as
accurate as possible
Kepler's 3 Laws of Planetary Motion
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1) The orbits of the Planets are ellipses, with the Sun at one focus of the ellipse
2) The line joining the planet to the Sun sweeps out equal areas in equal times as the
planet travels around the ellipse (see animation)
3) The ratio of the square of the revolutionary periods for two planets is equal to the ratio
of the cubes of their semi-major axes
Kepler finally devised a model of the solar system that predicted the position of the
planets with great accuracy
However, Kepler's model did not prove that Sun was at the center
Kepler's model lacked the observation evidence and the reason to explain why the Sun
was located at the center of the Solar System
Galileo's Discoveries
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Observations showed that Moon and Sun are not perfect
Moons orbiting around Jupiter - NOT all celestial objects orbited around the Earth
Venus exhibited a full set of phases - Definitive observational proof that the Sun is at the
center of the Solar System and NOT the Earth
Newton's 3 Laws
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1) Unless an object is acted upon by a net, outside force, the object will maintain a
constant speed in a straight line or remain at rest
2) A net external force applied to an object causes it to accelerate at a rate that is
proportional to the force and inversely proportional to its mass
3) For every action there is an equal and opposite reaction
Mass and Weight
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Mass - an intrinsic property of the object and remains the same independent of where it is
in the universe
Weight - downward force experience by an object as a result of the gravitation interaction
with the Earth or other body
Gravity
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In weak gravity (like on the Earth), Newton's Theory works well
Newton's Theory was only an approximation to the more exact Theory devised by
Einstein
Observations unexplained by Newton's laws of gravity: 1) the bending of starlight and 2)
the full precession of Mercury's orbit
Einstein's Theory of Gravity explains the effects seen in high gravity