Ancient Greek Universe Model

Overview

This lecture explains how ancient Greeks understood the structure of the universe, focusing on their geocentric (Earth-centered) model, celestial spheres, and the reasoning behind it.

The Location of Stars

• The ancient Greeks knew Earth was round and believed stars surrounded the Earth in all directions.
• Stars appear "up" from any point on Earth's surface.
• Based on the logic that gravity pulls things down, Greeks wondered why stars didn't fall.

The Celestial Sphere Concept

• Greeks thought stars were attached to a giant "ceiling" called the celestial sphere, surrounding Earth.
• The celestial sphere was held in place by Earth's gravity pulling inward from all directions.
• Stars were believed to be fixed in place on this sphere.

Why Stars Move Across the Sky

• To explain why stars rise and set, Greeks considered two options: Earth rotates or the celestial sphere rotates.
• Greeks concluded the celestial sphere rotates once per day, not the Earth, since Earth's movement wasn't felt.

The Sun and Additional Spheres

• The Sun moves through constellations and could not be fixed on the celestial sphere.
• Greeks added another transparent sphere (the "Sun sphere") to hold the Sun, spinning at a different rate.
• The Sun sphere was thought to be made of a clear substance (like glass or crystal) so it wouldn't block star views.

Spheres for Moon and Planets

• The Moon also needed its own sphere because it moves at a unique pace.
• Any celestial object moving differently (like planets) was given its own sphere.
• Planets ("wanderers" or "planeta" in Greek) moved through constellations and so each was attached to its own sphere.

Predictions and Model Adjustments

• This system allowed fairly accurate predictions of positions of stars, Sun, Moon, and planets.
• As star maps improved, the Greeks had to keep refining the system to match new observations.

Key Terms & Definitions

• Celestial Sphere — A giant, imaginary sphere surrounding Earth, with stars fixed upon it.
• Geocentric Model — A model of the universe with Earth at the center.
• Planeta — Ancient Greek word for "wandering star," referring to planets.
• Sun Sphere / Moon Sphere — Transparent spheres in the Greek model, each carrying the Sun or Moon, spinning at different speeds.

Action Items / Next Steps

• Review textbook diagrams of the Greek model with multiple spheres.
• Prepare to discuss how the geocentric model was eventually replaced.