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Historical Astronomy
History
Aristotle – Aristarchus Ptolemy
Who cares about Astronomy???
Hunters- Hunted animals who’s migration
depended on the seasons
Gatherers- Harvested fruit and nuts at
certain times of the year
Development of Agriculture- Important to
harvest crops at the right times of the year
Annual meetings of nomadic tribes based
on position of stars and planets
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F.D.G.
Famous Dead Guy
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Aristotle (384-322 B.C.)
Developed models to explain
the Universe without fully
understanding it.
Based his models on beautiful
crystalline spheres
Taught Earth was the
immobile center of the
Universe
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Aristotle (384-322 B.C.)
Crystalline
Spheres
Earth as center of
Universe
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Continued …
Aristotle’s model was believed for nearly
2000 years because no one observed
parallax of stars
– Concluded Earth did not move
Parallax: The change in the apparent position of an
object due to a change in the location of the
observer
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The problem of the unseen parallax
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No stellar parallaxes were detected
in ancient times
Two possible explanations
1. The stars are very far away, so that their
parallaxes were too small to measure in
ancient times
2. The earth did not go around the sun
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Aristarchus of Samos (310 – 230
B.C.)
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Aristarchus of Samos (310 – 230
B.C.)
Predicted that
the Sun was
very far away
and much
larger than the
moon and
Earth
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Aristarchus of Samos (310 – 230
B.C.)
Observed the Earth’s shadow on the moon
during a lunar eclipse
Thought it was absurd to assume that the
Sun revolved around so small a body
(Earth)
– First person to present a good argument for a
“sun at the center of the Universe” model!
Sometimes known as the “Greek Copernicus”
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Aristarchus of Samos (310 – 230
B.C.)
Believed in the “stars to far away to see the
parallax” theory
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Ptolemy (Claudius Ptolemaeus, ca.
90 – ca. 168 A.D.)
Lived in Alexandria, the center of learning
at the time
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Ptolemy
Continued to
support the
Geocentric
model of the
Universe
– Geocentric =
Earth centered
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Planetary Motion
Attempted to explain planetary motion with
his model of the Universe
Prograde Motion-The apparent forward
motion of a planet with respect to the
background stars
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Planetary Motion Cont…
Retrograde Motion-The apparent
backword motion of a planet with respect
to the background stars
– Examples: Mars, Venus, Uranus, many of
Saturn and Jupiters moons
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Retrograde Motion
Over a period of 10 weeks,
Mars appears to stop, back
up, then go forward again.
Understandable in a
heliocentric model / how
about in a geocentric model ?
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Increased Complexity
Additional need for accuracy in
predictions led to further and further
complexity of models, ultimately codified
by Ptolemy
Ptolemy adopted a “wheels within
wheels” approach to his geocentric
model of the Universe
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Wheels within Wheels
Epicycles: The smaller cirlces that move
along the larger circles called the deferent
Deferent: The larger circle that the smaller
epicycles move around
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Continued …
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Size of the Universe
Models of Aristotle / Ptolemy envisioned (required) a
relatively small universe
– Even the grandest, Ptolemy’s, put the rough size at about
50,000,000 miles (one light year = six trillion miles)
Why so small ?
– If earth does not move, the rest of the Universe must !
– Ptolemy’s universe would complete a cycle around the earth
each 24 hours
– Speed required ~ 10,000,000 mph !
– If larger, then speed larger still …
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