Historical Astronomy 10000BC-3000BC•   10000BC

– 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)

– 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

Greek Astronomy 500BC-350BC• 500BC

– 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

Greek Astronomy - Geometry• 300BC

– 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 EarthEratosthenes 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 Copernicus1473-1543 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)