Astronomy Historical Models and Theories

Study Notes

Classical Greece: Advocated a geocentric model where Earth is spherical, motionless, and at the universe's center.
Hipparchus: Introduced the concept of precession of the equinoxes, affecting the timing of seasons.
Eratosthenes: Estimated the Earth's circumference with remarkable accuracy.
Ptolemy: Developed explanations for retrograde motion but could not address all astronomical observations.

Eudoxus & Aristotle: Supported the geocentric model; failed to explain the brightness variations and size changes of celestial bodies.
Aristarchus: Proposed an early heliocentric model.
Galileo Galilei: Improved observational astronomy; discovered moon craters and Jupiter's moons, observed sunspots, and phases of Venus.
Copernicus: Advanced the heliocentric model; provided insights on retrograde motion but struggled with stellar parallax.
Tycho Brahe: Notable for accurate planetary positions; importantly found no evidence of stellar parallax.
Johannes Kepler: Formulated three laws of planetary motion:
- Law of Orbits: Planets move in elliptical orbits.
- Law of Areas: Equal areas are swept out in equal times (perihelion, aphelion).
- Law of Periods: Relationships between orbital periods and distances (𝑇 ~𝑎).
Isaac Newton: Established that gravitational laws explain elliptical orbits.

Newton's Law of Universal Gravitation: Characterizes the attractive force between bodies proportional to mass and distance.
Centripetal Force: An essential concept in understanding orbits.

General Theory of Relativity: Provided a new framework for understanding gravity as the curvature of spacetime caused by mass.

Differences:
- Model: A representation of reality that simplifies complex systems.
- Theory: A well-substantiated explanation of a phenomenon based on observed evidence.
- Law: A statement derived from repeated experimental results.

Must be elegant, contain minimal arbitrary elements, agree with existing observations, and make detailed predictions.

Logically sound, consistent, plausible results, capable of quantitative predictions, and subjected to continual testing and comparison.

An astronomical theory positioning Earth at the universe's center, with celestial bodies orbiting around it.

Aristotle: Advocated for a spherical Earth and the geocentric perspective, emphasizing perfect circular motion of celestial bodies.
Ptolemy: Developed the 2nd-century Ptolemaic system, introducing epicycles to account for the retrograde motion of planets.

Stationary Earth: The model holds that Earth remains fixed in the center of the cosmos.
Celestial Spheres: Concept of clear, rotating spheres that carry planets and stars around Earth.
Epicycles: Smaller circular paths in which planets move while following larger orbits around Earth.

The geocentric model significantly influenced medieval and Renaissance thought, embedding the idea of humanity's central role in the cosmos within religious beliefs.

The model faced criticism due to inaccuracies in predictions regarding planetary positions and movements, leading to a decline in its credibility.
It was eventually superseded by the heliocentric model proposed by Copernicus, which accurately positioned the Sun at the center of the solar system.

The geocentric model dominated astronomical thought for over a thousand years, forming a foundation for subsequent astronomical studies.
It played a crucial role in the evolution of astronomy, eventually paving the way for the acceptance of the heliocentric theory.