Overview
This lecture covers the historical development of our understanding of the solar system's structure, focusing on the shift from geocentric (Earth-centered) to heliocentric (Sun-centered) models and the key figures and discoveries that shaped modern astronomy.
Geocentric vs. Heliocentric Theories
- Geocentric theory places Earth at the center of the universe; heliocentric theory places the Sun at the center.
- Geocentric models struggled to explain planetary retrograde motion.
- Heliocentric models clarified planetary orbits and solved retrograde motion issues.
Early Observations and Cultures
- Ancient monuments like the Pyramids of Giza and Stonehenge show early interest in celestial patterns.
- Babylonians developed constellations, the 24-hour day, and lunar calendars.
- Ancient Greeks proved Earth was round and used geometry for astronomical calculations.
Greek and Roman Contributors
- Aristotle believed in the geocentric model, citing lack of observable parallax.
- Parallax is the apparent shift of an object's position due to the observer's movement.
- Aristarchus first proposed a heliocentric theory using geometry.
- Eratosthenes calculated Earth's circumference; Hipparchus measured Earth-Moon distance.
Ptolemy and Epicycles
- Ptolemy's geocentric model used epicycles (small circular orbits) to explain retrograde motion.
- Despite being incorrect, Ptolemy’s model accurately predicted planetary positions.
Copernicus and the Heliocentric Model
- Copernicus correctly placed the Sun at the center with planets orbiting it in circles.
- Retrograde motion is explained by Earth passing slower-moving outer planets.
- Introduced the concept of the astronomical unit (AU) and calculated planetary distances.
Brahe, Kepler, and Planetary Motion
- Tycho Brahe made precise observations of planets and stars, aiding later astronomers.
- Johannes Kepler used Brahe’s data to formulate three laws of planetary motion:
- Planets orbit the Sun in ellipses, with the Sun at one focus.
- Law of equal area: Planets sweep out equal areas in equal times, moving faster when closer to the Sun.
- Relationship between orbital period and distance from the Sun (P² = a³ for planetary orbits).
Galileo’s Observations
- Galileo used a telescope to observe lunar craters, sunspots, and the moons of Jupiter.
- He discovered Venus has phases, supporting the heliocentric model.
- His findings contradicted Church teachings and led to his persecution.
Newton’s Contributions
- Isaac Newton formulated the laws of motion and universal gravitation.
- Newton’s first law: An object in motion stays in motion unless acted on by an external force.
- Newton’s third law: Every action has an equal and opposite reaction.
- He invented a powerful reflecting telescope and expanded understanding of planetary and orbital motion.
Key Terms & Definitions
- Geocentric — Earth-centered model of the universe.
- Heliocentric — Sun-centered model of the solar system.
- Epicycle — Small circle used in geocentric models to explain retrograde motion.
- Parallax — Apparent shift in an object's position due to a change in the observer’s vantage point.
- Ellipse — Oval-shaped orbit with two focus points.
- Eccentricity — Measure of how much an orbit deviates from being a circle.
- Astronomical Unit (AU) — Average distance from the Earth to the Sun (about 150 million km).
- Retrograde Motion — Apparent backward movement of a planet across the sky.
Action Items / Next Steps
- Download and review the Unit 1 test review to identify key individuals and groups for the test.
- Prepare for further study on later astronomers (Hubble, Lemaître, Einstein, Rubin, Cannon, Hawking) in upcoming lectures.