The Copernican Revolution
Epicycles and Planetary Motion
More Sophisticated Epicycles: The Ptolemaic Universe
Aristotle’s Universe
 Geocentric (Ptolemaic) model—The earth is stationary (nonrotating) and located at the center of the universe. The
planets, the sun, and the moon revolve around it.
 The universe is divided into two regions—the sub-lunar
(inner) region, which includes everything inside the moon’s
orbit, and the super-lunar (outer) region, which extends from
the moon’s orbit to the sphere of the stars.
 There is no empty space. An incorruptible ether pervades the
entire super-lunar region.
 All motion in the super-lunar region is perfectly circular.
However, some of the circular motion is in the form of
epicycles.
 All objects in the sub-lunar region are mixture of the four
elements—earth, air, fire, and water. The proportion of those
elements determines each object’s natural place in the sublunar region.
 Each object has a natural propensity to move in a straight
line, upward or downward, towards its natural place.
 All motion except natural motion requires causation.
 There are four types of causes:
o material—the substance(s) that make up an object
o formal—the arrangement of an object’s parts; its “shape”
o efficient—the force(s) that moved an object to its current
state
o final—the goal or end that the object is moving toward
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Arguments Against Copernicus’s Heliocentric Model
Tower Argument
1. If the Copernican model were correct, then an object dropped
from the tower would land a significance distance from the
base of the tower.
 E.g., if a stone were dropped from the top of a 64-foot
tower, it would hit the ground in 2 seconds during which
time the tower would have moved approximately 2933
feet from its initial position. (This assumes that the earth
has a circumference of 24,000 miles and makes a complete
rotation once every 24 hours.)
2. An object dropped from a tower would land very close to the
base of the tower.
3. Therefore, the Copernican model is not correct.
Motion of Objects Near the Earth’s Surface
1. If the Copernican model were correct, then loose objects on
the surface of the earth would be flung away (like stones
from the rim of a rotating wheel).
2. Loose objects on the earth’s surface are not flung away.
3. Therefore, the Copernican model is not correct.
Motion of the Moon
1. If the Copernican model were correct, then the earth would
leave the moon behind as it (the earth) revolves around the
sun.
2. The earth does not leave the moon behind as it revolves
around the sun.
3. Therefore, the Copernican model is not correct.
Apparent Positions of the Stars
1. If the Copernican model were correct, then parallax would be
observed in the apparent positions of the stars in the sky.
2. Parallax is not observed in the apparent positions of the stars
in the sky.
3. Therefore, the Copernican model is not correct.
Apparent Sizes of the Planets as Viewed from the Earth
1. If the Copernican model were correct, then the apparent sizes
of the planets when seen from the earth would change
appreciable during the course of a year.
2. The apparent sizes of the planets when seen from the earth do
not change appreciably during the course of a year.
3. Therefore, the Copernican model is not correct.
Main Strengths of the Copernican Model
 It provided a simple, elegant explanation of the observed
motions of the planets—particularly their retrograde motion.
 It accounted for the apparent coordination of the motions of
the sun, Mercury, and Venus.
How did the Copernican model come to be accepted? Mainly
through the efforts of scientists who came after Copernicus—
Galileo, Kepler, and Newton.
Galileo
 (“father of modern science”)—most important defender of
the Copernican model:
 built improved telescope through which he saw—
o moons of Jupiter
o the surface of the moon, which contained craters,
mountains, etc.
o phases of Venus
 refuted the tower argument by noting that the object dropped
from the tower would also be in horizontal motion, which
would continue after it was dropped
 studied terrestrial mechanics
o developed mathematical formula for distance traveled by
freely falling bodies over periods of time; showed that
time/distance relationship is independent of the weight of
the falling body
o developed notion of inertia that was inconsistent with
Aristotle’s view that all non-natural motion requires a
cause
o studied projectile motion and showed that it could be
analyzed in terms of horizontal and vertical components
Johannes Kepler
 proposed the idea that planets’ orbits around the sun are
elliptical rather than circular
 developed laws of planetary motion that dispensed with the
need for epicycles and better matched observations of the
planets’ motion
Issac Newton
 unified the super-lunar and sub-lunar regions: the same laws
of physics apply to both regions
 clearly identified force as the cause of acceleration rather
than of motion
 developed principles of mechanics and gravitation that
subsumed Kepler’s laws of planetary motion and Galileo’s
laws of falling bodies
Question—How can falsificationists, like Popper, account for the
fact that scientists often hang on to their theories
even when they appear to have been falsified?
o Possible answer (Popper): On some occasions, dogmatic
allegiance to a theory on the part of scientists is
justified, even when the theory appears to be falsified.
o Criticism of answer (Chalmers): To accept dogmatism as
having a role to play in science greatly compromises
the criterion by which Popper and other
falsificationists distinguish between real science and
pseudo-science.