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Models of the
Solar System
Chapter 27
Section 2
Two Basic Models of the Universe
1. Geocentric –
The Earth is located in the center
and all other celestial bodies orbit
around the Earth.
2. Heliocentric The Sun is located in the center
and all other celestial bodies orbit
around the Sun.
Aristotle (384 – 322 BC)
• Greek philosopher
• Developed the first geocentric universe
(Earth-centered)
• Believed the Sun, Moon, five visible
planets (Mercury, Venus, Mars,
Jupiter, and Saturn), and the stars
orbited the Earth.
• By 1500s this theory was believed as fact.
Challenges to a Geocentric
Universe
• The 5 visible planets seem to wander in
the sky.
– Each evening, they move east, briefly
slow down, then reverse direction
(retrograde motion)
Claudius Ptolemy
• Greek astronomer
• Wrote the Almagest
• expanded Aristotle’s theory by
explained how the circular pattern of the
planet’s motion creates retrograde
motion depending on the size of the
circle.
• Was able to predict the locations of the
planets in the night sky.
Nicholaus Copernicus (1473-1543)
• Proposed a heliocentric model of the
universe (Sun centered)
• he theorized that the retrograde motion
of planets could be explained in a
heliocentric system as a result of
differences between the time it takes
each planet to orbit the Sun.
• His theories were dismissed
Johannes Kepler (1571 – 1630)
• Polish mathematician and astronomer
used the measurements of Tycho
Brache
• Theorized that the planets revolve
around the sun in elliptical orbits and
that the sun is not in the exact center
of the orbits.
• Suggested a force within the Sun that
has great effect on nearby objects
Galileo Galilei (1564-1642)
• 1609 constructed a small telescope
with which he could observe
mountains on the Moon, four Moons
orbiting Jupiter, and Venus goes
through phases like our Moon.
• His observations confirmed the
incorrect statements of a geocentric
universe.
• First to explain inertia – motion is
due to a force and a moving body
will stop or change direction only
in response to a force.
• Concluded a force pulls all falling
bodies to the earth – identified the
force as magnetism.
Isaac Newton (1642-1727)
• Reasoned that the force identified
by Galileo acts on an apple, so it
must also act on the moon.
• Developed the law of universal
gravitation – every body in the
universe attracts every other body
and that attraction is affected by
size and distance.