astronomers

geocentric model

• earliest model of solar system

• earth at the center

• needed lots of complications to accurately track planetary motions

heliocentric model

• sun is at center of solar system

• only moon orbits around earth; planets orbit around sun

pythagoras

• his ideas would influence generations of scientists and philosophers

• came up with the theory that Earth is a perfect sphere

  • true to some extent; equilateral bulge, earth is “pear-shaped”

  • devations from sphereicty: mountains, canyons, deep-sea trenches

eratosthenes

• conducted a brilliant experiment to determine size of earth

• a large obelisk in alexandria casts a ~7^° shadow when the sun is at its highest point in sky

• 5000 stadia (~500 miles) away in egyptian city of syene, the sun casts no shadow at highest point in sky

  • must be ~7^° between lines joining alexandria and syene to the center of the earth

aristarchus

• measured size of moon

• estimated distance to sun (wrong by factor of ~20)

  • proposed sun was center of the heavens

  • heliocentric model first appears

  • problem: should see stars shift positions (stellar parallax)

copernicus

• polish doctor & lawyer

• revisited ideas of aristarchus’ 2000 year old “heliocentric” model

  • now able to explain retrograde motion: planets on different, sun-centered orbits pass each other

  • published ideas before his death

    • first person to describe details of heliocentric model

  • Still some discrepancies…

    • model had perfectly circular orbits

    • still no parallax observed

tycho brahe

• danish nobility

  • utilized wealth to study the sky

• built most accurate pointing and measuring instruments of his time

  • still could not observe parallax; one of the last to hold onto geocentric model

• one of the first to establish importance of repeatability in scientific measurement

  • more data = more accurate results

kepler

• one of brahe’s assistants

  • inherited his work when he died

• superior data (by volume and precision) showed that mars’ orbit is not circular, but elliptical

  • noticed the sun was not at the center of the calculated orbit, but off to the side, at a “focus” of the ellipse

  • with this model, kepler calculated orbits of other planets, and found excellent agreement with actual observations

kepler’s 3 laws

1. planets move in elliptical orbits, with sun at one focus of the ellipse

2. planets do not move with constant speed. they move faster when nearer to sun, and slower when they are farther away

3. the amount of time it takes a planet to orbit the sun exactly once is related to size of orbit (semi-major axis)

   • P² years = a³ AU

galileo

• italian scientists, fascinated by motion

• used a telescope to:

  • observe and draw surface features of the moon

    • concluded moon was ball of rock

  • observe changing sun spots (sun not a constant orb)

  • observe 4 large moons of jupiter, proving some bodies in solar system did not orbit earth

  • observe evidence of saturn’s rings

  • observe phases of venus as proof of heliocentric model