Question

This geocentric model of the universe was created by Aristotle.

Explain how our current understanding of the universe is different from the model. Discuss at least 3 differences.

(6 points)

Answer 1

Aristotle’s geocentric model (Earth at center, everything else made of concentric perfect spheres) differs from our current understanding in several important ways. Three (plus one extra) key differences:

1) Center of the universe
- Aristotle: Earth is fixed at the absolute center; all planets, Sun, Moon, and stars circle Earth.
- Modern view: There is no special central point. Planets orbit the Sun (heliocentric); the Sun is one star among billions in the Galaxy, which itself is one of many galaxies in a vast, expanding universe.

2) Motion and orbits
- Aristotle: Celestial bodies move in perfect circles at constant speed on solid, nested spheres (circular uniform motion).
- Modern view: Planets follow Kepler’s laws — ellipses, not perfect circles — and their speeds vary along the orbit. Motion is explained by gravity (Newton/Einstein), not by attached crystalline spheres or a tendency to move in circles.

3) Nature of celestial bodies
- Aristotle: Heavens are made of a special unchanging fifth element (aether); stars are fixed to a single outer sphere and do not change.
- Modern view: Celestial bodies are made of the same kinds of matter and governed by the same physical laws as Earth. Stars are distant suns at varying distances; they evolve, and galaxies move and interact.

4) Explanation of forces and change (extra)
- Aristotle: Natural places and intrinsic tendencies explained why things fell or rose; celestial and terrestrial realms obeyed different rules.
- Modern view: Universal gravity (and general relativity) explains motion both on Earth and in space with one consistent set of physical laws; Earth rotates on its axis and orbits the Sun, explaining day/night and seasons.

Brief examples of empirical tests: phases of Venus and moons of Jupiter observed by Galileo support heliocentrism; stellar parallax and precise orbital measurements confirm Earth’s motion.