Models of the Universe — Kepler & Newton

This lecture traces how we figured out that Earth orbits the Sun. The Greek geocentric model (Aristotle, 4th c. BC) put Earth motionless at the center, with everything else circling on perfect spheres. Ptolemy (~140 AD) refined it using epicycles riding on deferents to explain retrograde motion — the temporary westward drift of planets. It worked predictively but required ~80 circles.

Copernicus (1543) revived the heliocentric model: Sun at the center, planets orbiting it. Retrograde motion now followed naturally — Earth simply overtakes a slower outer planet. But Copernicus kept circular orbits, so the model still needed minor epicycles.

Tycho Brahe spent 30 years collecting the best naked-eye planetary positions ever recorded. His assistant Johannes Kepler distilled the Mars data into three laws: (1) orbits are ellipses with the Sun at one focus; (2) a planet sweeps out equal areas in equal times, so it speeds up at perihelion and slows at aphelion; (3) P² = a³ in units of years and AU.

Galileo turned a telescope to the sky and found heavenly bodies are imperfect (lunar mountains, sunspots), Jupiter has its own moons, and Venus shows a full set of phases — possible only if Venus orbits the Sun. This was direct observational proof of heliocentrism.

Newton unified terrestrial and celestial physics. His three laws of motion plus the universal law of gravitation — F = G·m₁m₂ / r² — explain why an apple falls and why the Moon orbits Earth. Gravity is an inverse-square attraction between every pair of masses (G = 6.67 × 10⁻¹¹). Kepler's three laws emerge as mathematical consequences. With this, modern physics-based astronomy was born.