Ancient Cosmology

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Ancient Cosmology

• Monday, September 29

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For 2000 years, geocentric model for the universe
was widely assumed.
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Most famous advocate of geocentric model the
astronomer Ptolemy (2nd century AD).
Developed elaborate model to describe motion of
stars, Sun, Moon, planets.
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Stars
Circular motion around north celestial pole (near
the North Star, Polaris)
Time to complete circle 23 hours, 56 min. 1
sidereal day
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Stars look as if they were glued to rigid
spherical shell rotating once every 23 hr, 56 min.
(You cant tell, without a telescope, that
some stars are closer than others.)
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Sun
Circular motion around celestial pole part of
circle is below horizon, so we say Sun rises
and sets.
Time to complete circle 24 hours.
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Observed motions of the Sun humanitys first
clock.
Noon
Sunrise
Sunset
1 solar day time from one noon to the next 24
hours
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IF the Sun orbited the Earth once per day
Midnight
Sunset

Observer
Noon
Sunrise
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SINCE the Earth rotates once per day
Noon
Sunset
Midnight
Sunrise
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• Observed motions of the Sun can be described if
  either
• The Sun goes around the Earth once per day, or
• 2) The Earth rotates about its axis.

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Describing motions of stars and Sun (and also
Moon) was fairly simple in the geocentric model.
Describing motion of planets was difficult.
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Planet Wanderer in Greek
To naked eye, a planet looks like a star -
a tiny blob of light.
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Planets are distinguished by their motion
relative to stars.
? NORTH
Planets usually move west to east, but sometimes
east to west (retrograde), relative to stars.
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Ptolemys explanation of retrograde motion
Planet (P) moves in a small circle called the
epicycle.
Center of epicycle (A) moves in a large circle
called the deferent.
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Combination of small and large circles creates
loop-the-loop retrograde motion.
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Detailed structure of Ptolemys geocentric model
Complicated!
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A bold minority opinion
Aristarchus (3rd cent. BC) proposed that the
Earth rotates on its axis goes around the Sun.
First heliocentric (Sun-centered) model.
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Heliocentric model was rejected by the
contemporaries of Aristarchus.
Aristarchus was accused of impiety.
Why did Aristarchus bother with a heliocentric
model, given the grief he received?
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Questions posed by Aristarchus
How far away is the Sun?
How large is the Sun?
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How far away is the Sun?
Farther away than the Moon!
Moon comes between Sun and Earth during a
solar eclipse.
Moon, like Earth, is an opaque sphere, capable of
blocking light.
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Phases of the Moon The Moon is an
opaque sphere illuminated by the Sun.
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Gibbous, crescent shapes result of perspective.
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How Aristarchus found the relative distances of
Sun and Moon.
First quarter or last quarter Moon we see
bright portion of Moon as a perfect
half-circle.
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When we see the Moon as a half-circle,
Earth-Moon-Sun angle must be 90.
90
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When Aristarchus saw the Moon as a half-circle,
he measured the Moon-Earth-Sun angle to be 87.
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A more accurate diagram
M
S
E
Get out your rulers
Earth Sun distance (E-S) is 19 times
Earth Moon distance (E-M).
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Aristarchus Sun is 19 times farther away (and
thus 19 times bigger) than Moon.
(This is actually an underestimate of
the Suns immense size.)
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Aristarchus Earth is 3 times size of Moon (from
size of Earths shadow on Moon).
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Despite the arguments of Aristarchus (Why
should big Sun orbit tiny Earth?), heliocentric
model ignored for 18 centuries.
Reviver of heliocentrism Nicolaus Copernicus
(Polish 1473 1543)
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Wednesdays Lecture
Renaissance Cosmology
Reading
Chapter 2