Historical Astronomy 10000 BC3000 BC 10000 BC constellations

Historical Astronomy 10000 BC-3000 BC • 10000 BC – – constellations, lunar cycle, discovery of planets? calendar refinements for agriculture counting schemes months, year (in months, but uneven) • 3000 BC – # 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

• 500 BC Greek Astronomy 500 BC-350 BC – Pythagorus - concentric celestial spheres for Sun, Moon, planets - all bodies spherical (including Earth) – Philolaus - Earth goes around central fire (Sun) • 350 BC – 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 300 BC – 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

• 150 BC Greek Astronomy – Modern Foundations • Hipparcos - star catalog (850, position and brightness) • - better estimates of size and distance of Moon • R(earth)/R(moon)~8/3, D(moon)~60 R(earth), D(sun) big • - precession of Earth's pole • -epicycles and deferent (used by Ptolmey) to explain retrograde motion of planets • 150 AD • 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 20000 R(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)