Kepler formulated three laws of planetary motion based on his analysis of planetary observations. His first law states that planets orbit the Sun in ellipses with the Sun located at one focus. This supported the heliocentric model of the solar system by showing planets orbit the Sun rather than Earth. Galileo observed Jupiter's moons orbiting Jupiter and phases of Venus, providing further evidence against the geocentric model and supporting that other celestial bodies can be orbited besides Earth.

1. What is the solar system?
The solar system is the sun and all of the bodies that orbit the
sun. Our current model of the solar system is the sun-centered or
heliocentric (hee.leeooh.SEN.trik) model. In the heliocentric model,
Earth and the other planets orbit the sun. The earliest models for
the solar system assumed that the Earth was at the center of the
solar system, with the sun, moon, and planets circling it. These
models, which used Earth as the center, are called Earth-centered
or geocentric (je..oh.SEN.trik) models. The heliocentric model
was not generally accepted until the work of Copernicus and
Kepler in the late 16th to early 17th centuries.
Who proposed some earty models of
the solar system?
Until Galileo improved on the telescope in 1609, people observed
the heavens with the naked eye. To observers, it appeared that
the sun, the moon, the planets, and the stars moved around
Earth each duy. This caused them to conclude that Earth was not
moving. If Earth was not moving, then Earth must be the center
of the solar system and all other bodies revolved around it.
This geocentric model of the solar system became part of
ancient Greek thought beginning in the 6th century BCE. Aristotle
was among'the first thinkers to propose this model.
5 ldentify As you read the text,
underline the definitions of
geocentric and hetiocentric.
Research Use different sources
to research a geocentric modet ,
of thq solar syst.em from either
ancient Greece, dflcient China, or
Babyton. Write a short description
of the modet you choose.
Aristotle
Aristotle (AlR.ih.staht'1) was a Greek
philosopher. Aristotle thought Earth was the
center of uU things. His model placed the
moon, sun, planets, and stars on a series of
circles that surrounded Earth. He thought that
if Earth went around the sun, then the relative
positions of the stars would change as Earth
moves. This apparent shift in the position
of an object when viewed from different
locations is known as parattax (PAlR.uh.laks).
In fact, the stars are so far away that parallax
cannot be seen with the naked eye.
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50 Unit 2The Sotar System

2. &rfistarchus
Aristarchus (air.i.STAHR.kuhs) was a Greek
astronomer and mathematician. Aristarchus
is reported to have proposed a heliocentric
model of the solar system. His model,
however, was not widely accepted at the time.
Aristarchus attempted to measure the relative
distances to the moon and sun. This was a
major contribution to scieltce. Aristarchus's
ratio of distances was much too small but
was important in the use of observation and
geom etry to solve a scientific problem.
Arirfa rehus (abouf IX}*ZSABCE)
Aristotle,thought that:if, Edrth.6r- -
,moving, the positions
of the stars should change as Earth moved. ln fact, stars
are so far away that shiftb in their positions can only be
observed by telescope.
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7 Predict lf a star appears at position l during the summer,
during which season witt it appear at position 2?

3. Pfo$emy {*bsaef 3S#*g?# #S}
P&w&mmy
Ptolemy (TOHL.uh.mee) was an astronomer,
geographer, and mathematician who lived in
Alexandria, Egypt, which was part of ancient
Rome. His book, the Almage.sf, is one of the
few books that we have from t"hese early times.
It was based on observations of the ptranets
going back as much as 800 years. Ptolemy
developed a detailed geocentric model that
was used by astronomers for the next 14
centuries. He believed that a celestial body
traveled at a constant speed in a perfect circle.
In Ptolemy's model, the planets moved on
small circles that in turn moved on larger
circles. This "wheels-on-wheels" system fit
observations better than any model that had
come before. It allowed prediction of the
motion of planets years into the future.
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the solar system. Remember that :
during Ptolemy's time peopte i
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The Polish astronomer Nicolaus Copernicus
(nik.uh.LAY.uhs koh. PER.nuh.kuhs) felt
that Ptolemy's model of the solar system
was too complicated. He was aware of the
heliocentric idea of Aristarchus when he
developed the first detailed heliocentric model
of the solar system. In Copernicus's time,
data was still based on observations with the
naked eye. Because data had changed little
since the time of Ptolemy, Copernicus adopted
Ptolemy's idea that planetary paths should
be perfect circles. Like Ptolemy, he used a
"wheels-on-wheels" system. Copernicus's
model fit observations a little better than the
geocentric model of Ptolemy. The heliocentric
model of Copernicus is generally seen as
the first step in the development of modern
models of the solar system.
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of the sotar system differ from Ptolemy's
modet of the solar system?
Copernican lYla del
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5. II ldentify Underline text that summarizes Kepler's
three [aws.
t{ep[er
|ohannes Kepler (yoh.HAH.nuhs KEP.luhr) was
a German mathematician and astronomer. After
carefully an alyztng observations of the planets, he
reahrzed that requirirg planetary motions to be
exactly circular did not fit the observations perfectly.
Kepler then tried other types of paths and found
that ellipses fit best.
Kepler formulated three principles, which
today areknown as Kepler's laws. The first law states
that planetary orbits are ellipses with the sun at one
focus. The second law states that planets move faster
in their orbits when closer to the sult. The third law
relates the distance of a planet from the sun to the
time it takes to go once around its orbit.
12 Analyze How did Kepler's first law support the
idea of a hetiocentric sotar system?
perihelion
Gatfiteo
Galileo Galilei (gahl.uh.LAY.oh gahl.uh.LAY) was
a scientist who approached questions in the fashion
that today we call scientific methods. Galileo made
significant improvements to the newly invented
telescope. He then used his more powerful telescope
to view celestial objects. Galileo observed the moons
Io, Europa, Callisto, and Ganymede orbiting lupiter.
Today, these moons are known as the Galilean
satellites. His observations showed that Earth was ,.
not the only object that could be orbited. This gave
support to the heliocentric model. He also observed
that Venus went through phases similar to the
phases of Earths moon. These phases result from
changes in the direction that sunlight strikes Venus
as Venus orbits the sull.
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