Ancient Astronomy

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Ancient Astronomy
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Aristotle (384 322 B.C.E)
Greeks defined a natural order to the five known
elements Earth Water Air Fire and a fifth
element (Quintessence) which comprised the
heavens This arrangement of the elements
represented a Geocentric universe (or Earth
centered universe).
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The Aristotelian Universe All elements have a
natural state of rest in the following
order Earth occupies the center of the
universe Water rests upon Earth Air rests
upon Water Fire rests upon Air Then the realm
of the perfect heavens formed from the
Quintessence
http//honolulu.hawaii.edu/distance/sci122/Program
s/p7/p7.html
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Natural Motion vs. Violent Motion
Earth returns to Earth (inward natural
motion) Fire rises into the sky (outward
natural motion) Stars and Planets move around
the Earth (Circular perfect natural
motion) You throwing a rock (violent
unnatural motion of Earth) ALL OBJECTS HAVE A
DESIRE TO BE IN A STATE OF REST WITHIN THEIR
NATURAL UNIVERSAL POSITIONS
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Can you dispute this simple model? What happens
when you violently throw a stone into the realm
of air? What happens to the air you exhale
underwater? What happens when you light a fire?
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Aristotles Universe Geocentric Made up of 55
concentric crystalline spheres that held the all
of the objects within the heavens. All of the
spheres were set into constant motion by the
Prime Mover operating on the outermost sphere.
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Raphael depicts the Aristotelian universe with
the Earth in the center and the Heavens
(Quintessence) surrounding it. The persons
represent Aristotles Prime Moverthe necessary
heavenly impetus for the constant eternal motion
of the stars and planets around the Earth.
Prime Mover (Astronomy) 1509-1511 Vatican Stanza
della Segnatura, Rome
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A curious traveler peers through the edge of the
universe to catch a glimpse of the Prime Movers
utilities.
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• Greek views were, in a sense, the beginnings of
  empirical science
• Reasoning based on observation (though Greek
  reasoning was still yoked by scholastic
  teleology).
• Develop models strongly based on geometry to
  predict future behavior
• Provide debate in public forums
• For example

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Observation Based Reasoning For example The
Earth is Round
Greeks noted that the shadow of the Earth on the
moon suggests that the Earth is round. And the
shadow moves across the moon during a lunar
eclipse, thus the spherical moon travels in a
circular path around the spherical Earth and
moves through Earths shadow.
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Observation Based Reasoning For example The
Earth is Round
http//flora.aaas.org/curriculum/Prototype_0505/im
ages/ship_on_a_sphere.gif
The observation of southern constellations
falling below the southern horizon as one
traveled North (or vice versa). Also, the mast
of a ship is the last item to disappear from view
as the ship as the ship sails away.
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Geometric modeling of systems Radius of the Earth
Erastothenes in the year 200 B.C.E. Calculates
the Round Earths circumference by comparing
shadows at two locations
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Geometric modeling of systems Distance to the
Moon
Hipparchus measures the distance to the Moon
about 100 yrs after Erastothenes Earth radius
measurement.
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A key observation that supported the
Aristotelian geocentric universe was the lack of
what is known as stellar parallax. Stellar
parallax is a real visible phenomenon with modern
instrumentation, but in Aristotles day it was
imperceptible.
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One observation of the motion of celestial bodies
did not conform to the Aristotelian idea of
perfect constant circular motion. The planets
would sometimes appear to move in reverse during
various times a year. This motion is known as
RETROGRADE MOTION. Below is an example of
retrograde motion for the planet Mars over the
course of about 10 months. Aristotle did not
account for this!
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Claudius Ptolemy (c. 85-165)
In the second century A.D., Claudius Ptolemy of
Alexandria offered a model of retrograde motion.
His model preserved the accepted notion of
perfect circular motion of the heavens AND
preserved Earths obvious central location within
the universe.
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Epicycles Deferents
Ptolemys epicycles also provided a mechanism for
a planets observed varying brightness by varying
the distance between the Earth and the
planet. Right or wrong, it is Outstanding
Empirical Science!
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Birth of Modern AstronomyThe Copernican
Revolution
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Nicolaus Copernicus (1473-1543)
Polish cleric and astronomer Published De
Revolutionibus Orbium Coelestium (On the
Revolution of Celestial Orbs) in 1543 His book
argued for a HELIOCENTRIC UNIVERSE
(Sun-centered)
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Copernicus heliocentric model, or sun-centered
model was a guess based on the idea that epicycle
and deferent motion was too complicated. As
published in his De Revolutionibus, the Earth was
now just one of six planets that orbited the Sun.
Note that the system implies a center of the
universe (the Sun) and that the orbits of the
planets (wandering stars) are perfectly circular.
http//www.astro.uiuc.edu/projects/data/Retrograde
/
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Click on the link below to see the Copernican
explanation of retrograde motion
Copernican Retrograde Motion
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Though Copernicus could not see the celestial
wanderers with enough clarity to conclusively
validate his heliocentric system, there is a
test. If Venus was confined to
an epicycle orbiting the Earth AND stayed close
to the Sun as depicted above, then there should
be no visible phases of Venus. Meaning that
Venus should ALWAYS appear as a small crescent or
dark, however...
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If Venus moved around the Sun as shown below,
then there should be visible phases of Venus as
viewed from our Earthly perspective!
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Galileo Galilei (1564-1642)
Italian Mathematician Considered the father of
modern science Incorporates mathematical
modeling (not limited to simple geometry and
perfection) into his ideas of mechanics (study of
motion)
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Detailed studies of motion and acceleration of
objects Using inclined planes and free-falling
bodies Galileo argues against the Aristotelian
concept which suggests that rest is the natural
state of an object Galileo suggest that an in
the absence of friction, an object will continue
moving forever!! His studies in motion were
published 1638
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Subscribes to the Copernican beliefs Is the
first to use a telescope to study the
heavens Discovers the moons of Jupiter which
suggest that things can orbit objects other than
the Earth
The Galilean moons Callisto, Ganymede, Europa,
Io orbit around the the planet Jupiter
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Sees many stars that are undetectable without the
aid of magnification Witness to imperfections
(sunspots) on the perfect globe of the Sun (an
endeavor that practically blinds him in his later
years)
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Armed with his telescope, Galileo confirms the
phases of Venus. CONCLUSIVE evidence that (at
the very least) Venus and Mercury orbit the Sun.
All of his celestial discoveries were published
by him in books Sidereus Nuncius (Starry
Messenger) (1610) and his Dialogues concerning
the Two Chief World Systems (1632)
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And lo, with such rich and glorious rewards the
church did honor this brilliant mans
achievements.