Overview
This lecture traces the evolution of solar system models from the geocentric view to the heliocentric model, explaining key astronomical phenomena and their cultural impact.
The Geocentric Model
- The geocentric model placed Earth at the center of the solar system for centuries.
- Ptolemy improved the geocentric model by introducing epicycles (small circles superimposed on larger orbits) to explain retrograde motion.
- Retrograde motion is when a planet appears to move backward in the sky for a short time.
The Heliocentric Model and Copernican Revolution
- Copernicus revived the heliocentric model, placing the Sun at the center of the solar system.
- The heliocentric model easily explains retrograde motion as Earth overtaking outer planets in its orbit.
- Copernicus estimated planetary distances from the sun with geometric calculations.
Parallax and Evidence for Heliocentrism
- Critics argued that if Earth orbits the Sun, the position of stars should shift (stellar parallax).
- Stellar parallax is very small due to vast star distances and requires precise telescopes to detect.
- Parallax is measured by observing a star from different points in Earth’s orbit and using trigonometry to calculate its distance.
Cultural and Philosophical Implications
- The Copernican model challenged the idea that Earth and humanity are the center of the universe.
- Giordano Bruno expanded on these ideas, suggesting stars are suns with their own planets.
- The Catholic Church opposed these views; Bruno was executed for heresy.
- The scientific revolution faced resistance but helped advance freedom of thought.
Key Terms & Definitions
- Geocentric Model — Solar system model with Earth at the center.
- Epicycle — A small circle on which a planet moves, itself moving along a larger orbital circle.
- Heliocentric Model — Solar system model with the Sun at the center.
- Retrograde Motion — Apparent backward movement of a planet as seen from Earth.
- Parallax — Apparent shift in position of an object when viewed from different locations.
- Astronomical Unit (AU) — Distance from Earth to the Sun (≈ 150 million km).
Action Items / Next Steps
- Review trigonometry principles, especially regarding right triangles and angle calculations.
- Continue with the next lecture to learn how astronomers developed the Copernican model further.