Early Astronomy

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Early Astronomy
Lecture 3
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Thales
(624 - 546 B.C.)
Thought the earth was a flat disk floating on an
endless expanse of water and that all things
come from water.
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Anaximander (560 480 B.C.)

• First to propose that the Earth
• floats in empty space.
• Thought the Earth was surrounded
• by a crystalline sphere and all the
• stars were attached to this sphere.
• Believed the Earth was a cylinder
• with a diameter 3 times its height.

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Pythagoras (560 480 B.C.)

• First person to realize that the
• morning star and evening star were
• the same object (Venus).
• First person to postulate that the
• earth is a sphere.
• Added more crystalline spheres to
• Anaxmander's model (1 for each
• planet).
• Vibrations of spheres rubbing
• together created a harmonious
• music of the spheres.

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Plato (428 348 B.C.)

• Believed the Earth was the
• center of the universe.
• Heavenly motion must consist
• of perfect circles and proceed
• at constant speed.
• Celestial objects reside on huge
• spheres.

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Eudoxus (400 347 B.C.)

• Noticed Platos model had discrepancies and
  modified it with a
• mathematical description.
• Geocentric model with 27 crystalline spheres

• 1 for fixed stars
• 3 each for the sun moon
• 4 each for the 5 planets

First model to address retrograde motion of the
planets.
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Retrograde Motion
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Retrograde Motion
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Aristotle (428 348 B.C.)

• Celestial spheres were interconnected
• like gears of a giant machine.
• Gravity caused the earth and all heavy
• things to reside at the center while
• lighter things floated to the heavens.
• Dirt, rock and water collected at the
• center, forming a spherical Earth.
• Aristotles geocentric model of the
• Universe dominated Western thought
• for almost 2000 years!

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Aristarchus (310 230 B.C.)

• Developed the radical idea that the
• Earth and planets went around the
• sun.
• Scientifically measured the distance
• of the sun from the earth using
• modern geometry.
• First person in history to have a
• correct understanding of the solar
• system.
• Unfortunately, nobody believed him!

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Eratosthenes (276 196 B.C.)

• First person to measure the size
• of the Earth.

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Apollonius (240 190 B.C.)

• Tried to explain the retrograde motion
• of the planets with epicycles.

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Hipparchus (190 120 B.C.)

• Greatest Greek astronomer.
• Compiled a catalog of stars classified
• according to brightness (magnitude).
• Discovered precession of the equinoxes
• by comparing observations with
• Timocharis 150 years earlier.
• Accurately measured the distance to
• the moon using modern geometry.

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Library of Alexandria
Doors that lead to the storage rooms.
Storage room.

• Worlds greatest storehouse of knowledge and
  center of research in
• ancient times (established in 300 B.C.).
• One of the reasons why Greek society rose to
  intellectual preeminence.
• Existed for over 700 years before it was
  destroyed.

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Library of Alexandria
History is full of people who out of fear, or
ignorance, or lust for power have
destroyed knowledge of immeasurable value which
truly belongs to us all. We must not let it
happen again. - Carl Sagan
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Ptolemy (100 170 A.D)

• Refined the geocentric model of
• Apollonius.
• There was good agreement
• between the Ptolemaic model
• and observations.
• The Ptolemaic model was
• believed for the next 1,500
• years!

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Copernicus (1473 - 1543)

• 1,300 years after Ptolemy, Copernicus
• reinvents the idea of a heliocentric
• solar system.
• Copernicuss model explains
• retrograde motion in a much simpler
• manner.
• He was able to calculate the true
• orbital periods of the planets.
• The Copernican model used circular
• orbits and matched observations
• fairly well (only slightly off).

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Tyco Brahe (1546 - 1601)

• Set out to find a better model to
• satisfactorily explain planetary
• motion.
• Using only a sextant and a compass,
• he conducted the best naked-eye
• observations ever made!
• Still had trouble finding a satisfactory
• model so he hired a brilliant German
• astronomer, Keplar, to assist him.
• On his deathbed, Tyco begged Keplar
• to find a satisfactory model so . . .
• that it may not appear I have lived
• in vain.

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Johannes Keplar (1546 - 1601)

• Believed Copernicus was correct.
• Worked very hard to see if Tycos
• data matched with Copernicuss
• theory.
• After years of frustration and
• difficult calculations he realized
• that the orbits cannot be perfect
• circles. The orbits were elliptical.
• Discovered 3 beautiful mathematical
• laws of planetary motion.

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Galileo Galilei (1564 - 1642)

• Provided some of the strongest
• arguments for the Copernican
• viewpoint and led the scientific
• community to accept the Copernican
• viewpoint. This is now known as the
• Copernican revolution.
• First person to use a telescope for
• astronomy.
• Discovered Sun spots.
• Discovered 4 moons orbiting Jupiter.
• Used observations of the phases of
• Venus to prove that Venus orbits the
• Sun.

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Phases of Venus
The phases of Venus demonstrated that the
Copernican view must be correct.
James J Marie, Astronomy 2005