Astronomers

Pythagoras

• His ideas would influence generations of scientists and philosophers for centuries

• The solid body we call the “sphere” is perfect.

  • So naturally, the Gods would model the earth as a sphere during the Creation…

• True, to some extent…

  • Equatorial bulge…Earth is “pear-shaped”

  • Small-scale deviations from sphericity…mountains, canyons, deep-sea trenches, etc.

Eratosthenes

• Brilliant experiment to determine the size of the earth

• A large obelisk in Alexandria casts a ~7^° shadow when the sun is at it’s highest point in the sky…

• 5000 stadia (~500 miles) away in Egyptian city of Syene, the sun casts no shadow at it’s highest point in the sky…

  • There must be ~7^° between the lines joining Alexandria and Syene to the center of the (spherical) earth

Aristarchus

• Measured size of moon

• Estimated distance to Sun (but 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…

    • But still some discrepancies…

      • Model had perfectly circular orbits

      • Still no parallax observed

  • Published his ideas shortly before his death

    • First person to describe the details of the heliocentric model

Tycho Brahe

• Danish Nobility

  • utilized his wealth to study the sky

• Built most accurate pointing & measuring instruments of his time

  • Still could not observe parallax → one of the last to hold on to the geocentric model

• Actually, built at least 4 of each, operated simultaneously, directed by Brahe himself

  • 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

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

  • Proceeding with this model, Kepler calculated the orbits of the other planets, and found excellent agreement with the actual observations!!

Galileo

• Italian scientist, fascinated by motion 

• He used telescope to:

  • Observe and draw surface features of the moon

    • Concluded moon was ball of rock

  • Observe changing sunspots (sun NOT a constant orb…)

  • Observe 4 large moons of Jupiter, proving that some bodies in the solar system did NOT orbit the earth

  • Observe evidence of Saturn’s rings

  • Observe phases of Venus as proof of the heliocentric model

Newton

Established the Law of Universal Gravitation. Concurred that the Earth rotates around the Sun. Provided rationale that explained Kepler's three laws. Newton's three laws of motion and law of gravitation provided a solid foundation for understanding planetary motion.