Study Notes on Greek Astronomy and Earth's Shape

Introduction to Greek Astronomy

• Early Greeks believed Earth was stationary (geocentric view).
• Stars traveled in a celestial sphere around Earth.

Evidence for Spherical Earth

• Most Greeks accepted Earth was round around 500 B.C.
  • Pythagoras: First to propose spherical Earth.
  • Anaxagoras: Observed circular shadow on the Moon during lunar eclipses.
  • Aristotle: Cited multiple observations for Earth's sphericity, such as positions of stars, shapes of celestial bodies, and disappearing ships over the horizon.

Key Contributions of Greek Astronomers

Eratosthenes

• Calculated Earth's circumference (~40,000 km) using shadows observed in Alexandria and Syene during summer solstice.

Anaxagoras

• Explained lunar phases; moon shines by reflected sunlight.

Eudoxus

• Proposed fixed spheres for celestial bodies.

Aristotle

• Presented arguments for a spherical Earth; influential in natural sciences.

Aristarchus

• First to suggest a heliocentric view of the universe; measured distances to celestial bodies, albeit with errors.

Hipparchus

• Cataloged stars and predicted eclipses; noted for measuring the length of the year.

Claudius Ptolemy

• Developed Ptolemaic model with Earth at the center, incorporating epicycles for planetary motion.

Ptolemaic Model

• Described retrograde motion of planets using small circles (epicycles) around larger orbits (deferents).
• Kept geocentric perspective prevalent until the 1400s.

Transition to Heliocentric Model

Copernicus

• Revived heliocentric model, simplifying celestial mechanics. He justified this model mathematically by assuming Earth's rotation and revolution around the Sun.

Major Takeaways

• Greek astronomers used observation, geometry, and logic to deduce the nature of the universe, laying groundwork for future scientific methods.
• Key conclusions: Earth's spherical shape derived from lunar eclipse shadows, ship disappearances, and observations of celestial bodies' movements.