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22.1 Early Astronomy
Ancient Greeks
• Astronomy is the science that studies the universe
• Deals with properties of objects in space & the laws
under which the universe operates
• Aristotle – Earth is round bc it always casts a curved
shadow when it passes btw the sun & moon
• Eratosthenes – established the size of Earth
• Hipparchus – star catalog
Geocentric Model
• Earth was a sphere that stayed motionless at
the center of the universe
• The moon, sun, & the known planets –
Mercury, Venus, Mars, & Jupiter – orbit Earth
• Beyond the planets were hollow space, this is
were the stars traveled daily around the Earth
• Not a correct model
Heliocentric Model
• Sun centered model
• Aristarchus was the 1st to believe this model
• Earth & other planets orbit the sun
• Although there was evidence for this model,
the Earth centered model dominated Western
thoughts for about 2000 yrs
Ptolemaic System
• Claudius Ptolemy
• Presented a model of the universe
• Accounted for the movements of the planets
• Retrograde Motion results in a combination of
the motion of Earth & the planet’s own
motion around the sun
The Birth of Modern Astronomy
• Nicolaus Copernicus
• Tycho Brahe
• Johannes Kepler
• Galileo Galillei
• Sir Isaac Newton
Nicolaus Copernicus
• From Poland
• Convinced that Earth is a planet just like the other five
that were known
• Daily motions of the heavens could be better explained
by a rotating Earth
• Changed the model of the solar system to having the
sun at the center & the planets (Mercury, Venus, Earth,
Mars, Jupiter, & Saturn) orbiting around it
Tycho Brahe
• Born 3 years after the death of Copernicus
• Designed & built instruments called pointers
which were used for 20 yrs to measure the
locations of the heavenly bodies
• Brahe’s observations, esp. of Mars, were far
more precise than any that were previously
• Johannes Kepler was his assistant
Johannes Kepler
• Discovered 3 laws of planetary motion (pg. 618)
• Discovered that the orbit of Mars around the sun
was an oval shaped path = eclipse
• Mars approaches the sun = speeds up
• Mars moves away from the sun = slows down
Galileo Galilei
• Descriptions of the behavior of moving objects
• Constructed his own telescope
• Discovery of 4 satellites or moons orbiting Jupiter
• Discovery that the planets are circular disks
• Discovery that Venus has phases just like the moon
• Discovery that the moon’s surface was not smooth
• Discovery that the sun had sunspots, or dark regions
Sir Isaac Newton
• Was the 1st to formulate & test the law of
universal gravitation
• Inertia
• Gravitational force decreases w/ distance
• Greater the mass of the object, the greater its
gravitational force
Location of Earth
Location of Sun
Supporters of
Geocentric Model
Center of Universe
Heliocentric Model
Center of Universe
• What produces the retrograde motion of Mars?
• What major change did Copernicus make in the
Ptolemaic system? Why was this change so
• In your own words summarize Kepler’s 3 lays of
planetary motion.