Download Models of the Solar System - Middle School Science & Algebra I

Models of the Solar System
Earliest Astronomers (Before 400 BC)
► Early
civilizations (e.g.,
Maya, Babylonians)
observed the heavens
for religious and
political reasons, but
mostly for farming.
► They made very
accurate observations
and predictions, but
had no rational
explanations for what
they saw.
Aristotle (383-323 B.C.)
► Aristotle’s
model was
influenced by his ideas
of motion: things on
Earth can change and
tend to fall towards
the center, while
things in the heavens
are eternal and never
change their motion.
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Aristotle’s model was geocentric; the earth is in the middle
of the universe, and the sun, moon, planets and stars
revolve around it.
Everything in the heavens is perfect and unchanging.
The celestial bodies are attached to transparent crystal
spheres, one inside the other, which rotate around the
earth.
Aristarchus of Samos (ca. 275 B.C.)
►
Aristarchus believed in a heliocentric model: the sun is in
the middle of the solar system, and the planets, including
earth, revolve round it. Only the moon revolves around the
earth, and is closer than the sun, which is immensely far
away. The stars are even farther away than the sun.
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Unfortunately, almost no
one believed Aristarchus;
they went for Aristotle’s
model instead.
Aristarchus’s model had
two major weaknesses:
The earth did not appear
to move
Parallax of the stars
could not be observed
The geocentric model
was taught as truth for
the next 1700 years.
Ptolemy (ca. 125 A.D.)
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Ptolemy’s model was geocentric like Aristotle’s,
but had some improvements that fixed some
things about Aristotle’s model that did not agree
with actual observations:
The planets seemed to slow down and speed up
as they passed through different parts of the
zodiac (variable periods)
Some planets, notably Mars, appeared to
actually go backwards at times (retrograde
motion)
► Ptolemy
solved these
problems by adding
deferents and
epicycles to the orbits
of some of the planets
to help account for the
odd motions that did
not agree with the
geocentric theory.
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During the Middle Ages and early Renaissance in Europe,
the Church eagerly accepted Ptolemy’s model, because it
matched their science, but more importantly because it
placed humans in the center of an orderly universe created
by a rational, compassionate God.
Nicolaus Copernicus (1473-1543)
Copernicus was a priest who was
given the job of making a map of
the motion of the sky to help out
with a new calendar.
► He rediscovered the heliocentric
model around 1515, when he
decided it just made more sense
logically, and matched his
observations better.
► He was uneasy about his discovery,
though, and did not publish his
book until just before his death.
► Copernicus’s model does not
explain the orbits or the motions of
the planets, it just describes them;
it is a purely geometric model.
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Galileo (1564-1642)
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Galileo was the first astronomer with a telescope, and a
strong supporter of Copernicus’s model.
He made several important observations that supported
the heliocentric model:
The moon has mountains and valleys
Venus and Mercury have phases just like the moon,
which can only be explained if they orbit the sun and not
the earth
He discovered four moons of Jupiter
He discovered the Milky Way, which suggested that the
universe is much larger than people thought.
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Galileo’s loud (and
arrogant) support for the
heliocentric model were
not welcomed by some in
the Church; in 1633, he
was accused of heresy, his
works were condemned,
and he was placed under
house arrest for the rest of
his life. His life was only
spared because he agreed
to keep quiet.
Johannes Kepler (1571-1630)
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Kepler improved the heliocentric model by developing laws of planetary
motion:
Law of elipses: the orbits of the planets are elliptical, not perfectly round
Law of equal areas: planets sweep out equal areas in equal times.
Harmonic Law: The square of a planet’s orbital period is equal to the cube
of its average distance from the sun. (translation: planets close to the sun
go faster than ones that are far away because gravity is weaker far away)