Historical Astronomy 10000BC-3000BC
• 10000BC
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constellations, lunar cycle, discovery of planets?
calendar refinements for agriculture
counting schemes
months, year (in months, but uneven)
• 3000BC
– # of days in year: ~360
• (360 degrees in circle)
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heliacal rise of Sirius in Egypt -> 365 days
celestial pole
modern constellations (from Med. sailors)
solstices, equinoxes, Astrology
astronomical monuments (Stonehenge, Pyramids, etc.)
astronomy strong in Mesopotamia/Europe, China, Africa,
Polynesia, Americas: everywhere!
Ancient Astronomy around the World
• 500BC
Greek Astronomy 500BC-350BC
– Pythagorus - concentric celestial spheres for Sun,
Moon, planets - all bodies spherical (including Earth)
– Philolaus - Earth goes around central fire (Sun)
• 350BC
– Aristotle - Sun is further than Moon (slower against
stars), eclipses - Earth is round (shadow on Moon),
going north makes pole star rise
– choose geocentric model
• feels like it; no stellar parallax
• 300BC
Greek Astronomy - Geometry
– Aristarchus - size of Sun and Moon relative to Earth, relative distances (use
of geometry to deduce them)
– using geometrical reasoning
D(sun)/D(moon)~20 , R(earth)/R(moon)~20 , R(sun)/R(earth)~7
– Sun is much bigger, so choose heliocentric model (doesn't take hold,
Aristotle wins)
The Size of the Earth
Eratosthenes determines the true size of the Earth
(and gets it right)
using the day when the Sun
shines right down a well at
the solstice
Greek Astronomy – Modern Foundations
• 150BC
• Hipparcos - star catalog (850, position and brightness)
• - better estimates of size and distance of Moon
• R(earth)/R(moon)~8/3, D(moon)~60R(earth), D(sun)
big
• - precession of Earth's pole
• -epicycles and deferent (used by Ptolmey) to explain
retrograde motion of planets
• 150AD
• Ptolmey - worked out a full geometric geocentric
cosmology
• - accounts for retrograde motion of planets
• - predicts planetary positions
• - 55 concentric cosmic spheres, all circular motion
• (size of Universe about 20000R(earth))
Ptolemy’s Model
The Copernican System
Nicolas Copernicus
1473-1543 Poland
Ptolemaic system was looking
shaky because better
observations (Arab) showed it
kept missing the planetary
positions.
Copernicus recognized that a heliocentric
explanation was much simpler (but he just
thought of it as asthetically better). It didn’t
actually predict the positions better.
The Church was invested in the Ptolemaic system (tradition), and
Copernicus waited until his death to publish. Most didn’t believe him.
Galileo (1564-1642)
• Professor, engineer, scientist, writer, “heretic”
– One of the first to use experiment to deduce physical laws
• Laws of motion, velocity, acceleration, inertia, pendulums, falling
bodies
– Brought telescopes to Astronomy
– After initial skepticism, adopted Copernican model because of
empirical evidence in support
Galileo’s
discoveries
Celestial bodies are not perfect;
mountains on Moon, sunspots
The Earth is not the only center of
rotation (moons of Jupiter)
Venus goes in front of and behind
the Sun (can’t happen if Ptolemaic
system is right)