Astronomy - Copernicus, Tycho, & Kepler

Ancient Astronomy

  • Human cultures recognized the cyclic nature of celestial motions in the Stone and Bronze Ages.
  • Monuments like Newgrange (Ireland, ~3200 B.C.) and Stonehenge (3000-1800 B.C.) show astronomical alignments, possibly used as calendars or for eclipse prediction.
  • Newgrange: Sunlight illuminates the central chamber during the winter solstice.
  • Stonehenge: Aligned with sunset/sunrise and moonset/moonrise at solstices.

Greek Astronomy

  • Early written records on astronomy come from ancient Greek philosophy.
  • Greeks aimed to understand and describe celestial motions using mathematical models, not physical ones.
  • Models were often inaccurate due to flawed "first principles":
    • Geocentric Universe: Earth is at the center.
    • "Perfect Heavens": Celestial bodies move in perfect shapes (spheres or circles).
  • Greeks assumed Earth was stationary because they didn't observe parallax.
  • Aristotle's Cosmos:
    • World made of Earth, Water, Air, Fire.
    • Natural motion towards Earth; violent motion requires force.
    • Heavens made of "Quintessence," experiencing circular motion without change.
    • Earth is round, with the Moon revolving around it.

Aristarchus of Samos

  • Proposed a heliocentric system, which faced opposition.
  • Objections to heliocentrism included:
    • Why don't we feel Earth's motion?
    • Why doesn't the Moon get left behind?
    • Why don't we see stellar parallax?

Planetary Motion

  • Planets:

    • Move relative to fixed stars.
    • Vary in brightness and speed.
    • Exhibit retrograde motion.
    • Difficult to describe in an Earth-centered system.

  • Prograde Motion: Apparent West to East movement compared to background stars.

  • Retrograde Motion: Apparent East to West movement compared to background stars.

Ptolemaic Model

  • Ptolemy attempted to explain retrograde motion.
  • Planets orbit on deferents (orbits) and epicycles (orbits on orbits) around Earth.

Philosophical Ideas

  • The heavens represent perfection and immutability.
  • The circle is the perfect shape, dictating circular heavenly motions.

Epicycles

  • Ptolemy (100-170 C.E.) used epicycles to improve geocentric models.
  • Planets attached to small circles (epicycles) rotating on larger circles (deferents) centered on Earth.
  • Visualized as a planet on a Frisbee attached to a bicycle wheel with Earth at the center.
  • Fairly reproduced retrograde motion.
  • Ptolemy's model required many epicycles to accurately track planetary motions, leading to complexity.

Copernicus

  • By Copernicus's time (1473-1543), Ptolemaic model predictions were inaccurate.
  • Copernicus proposed a Sun-centered system for a more natural explanation of retrograde motion.
  • He retained circular orbits, requiring epicycles, making his model no simpler or more accurate than Ptolemy's.
  • Many favored the Copernican model for its aesthetic appeal.

Heliocentric Model

  • Retrograde motion is an apparent motion caused when one planet moves from being behind another planet to being in front of the other planet.

Copernicus's System

  • Earth is a planet.
  • Day/night due to Earth's rotation; year due to Earth's revolution around the Sun.
  • The Moon orbits Earth.
  • Explained retrograde motion elegantly.
  • Explained why Venus and Mercury are always near the Sun.
  • Provided a straightforward way to determine the scale of the solar system.

Problems with Copernicus’s System

  • Predictions of planetary positions were no better than Ptolemy’s because it still used perfect circles.
  • If the Earth is moving why don’t we feel it?
  • If the Earth is a planet, the other planets must be like Earth. Are they?
  • Why don’t the stars appear to shift as the Earth changes position?

Galileo Galilei

  • Used a Dutch-designed telescope to make startling observations that disproved ancient beliefs.
    • Sunspots: The Sun is not a perfect sphere.
    • Craters on the Moon: The Moon is not a perfect sphere.
    • Four moons of Jupiter: Not everything revolves around the Sun.
    • Observed the rings of Saturn
    • Phases of Venus: Venus passes through all phases, just as the Moon does. In a geocentric model, the phases of Venus were limited to crescents.
  • Founded the experimental method for studying scientific problems.

Galileo's Observations

  • Moon has mountains, valleys, and craters.
  • The Sun has imperfections and rotates.
  • Jupiter has moons.
  • Venus has phases.
  • Saturn has rings.
  • These contradicted the belief that heavens were constant and immutable.

Discoveries of Galileo

  • Phases of Venus (including “full Venus”), prove that Venus orbits the sun, not the Earth!
  • Venus is smallest when it is in the full phase and largest when it is in the new phase. Then Venus must be very far from Earth when it is in the full phase and quite closes to Earth when it is in the new phase – which supports the argument that Venus is orbiting the Sun not Earth.