Ancient Greeks and Earth's Shape

• The ancient Greeks were among the first to identify that the Earth is spherical, using observations and logical reasoning.

How the Greeks Knew the Earth is Spherical

• Around 500 B.C.E., most Greeks believed the Earth was round, not flat.

Pythagoras (c. 570-495 BCE)

• Greek mathematician and philosopher, best known for the Pythagorean Theorem.
• Pythagoras founded Pythagoreanism which emphasized mathematics, harmony, and the belief that numbers govern the universe.
• Pythagoras and his pupils first proposed that the Earth was spherical, based on geometry and the symmetry of celestial bodies.
• Pythagoras believed the sphere was the perfect geometric shape and that the gods would only create a “perfect” world, rather than through scientific observations.

Anaxagoras (c. 500-428 BCE)

• Greek philosopher and scientist who introduced the concept of Nous as a fundamental force organizing the cosmos.
• Anaxagoras explained natural phenomena using rational thought rather than mythology.
• Anaxagoras supported Pythagoras' proposal through observations of the shadows that the Earth cast on the Moon during a lunar eclipse, which were circular.

Aristotle (384-322 BCE)

• Greek philosopher and scientist, a student of Plato and tutor to Alexander the Great.
• Aristotle contributed to philosophy, physics, biology, and ethics.
• Aristotle provided multiple proofs that the Earth is spherical
• Around 340 B.C.E., Aristotle listed several arguments for a spherical Earth, which included:
  • The positions of the North Star
  • The shape of the Moon and the Sun
  • The disappearance of ships over the horizon

North Star

• Greeks noticed that when they traveled to places nearer the equator, like Egypt, the North Star appeared closer to the horizon.

The Shape of the Sun and Moon

• Aristotle argued that if the Moon and the Sun were both spherical, then perhaps the Earth was also spherical.
• During a lunar eclipse, the Earth's shadow on the Moon is always circular, which can only be caused by a spherical object casting the shadow.

Disappearance of Ships

• The Greeks observed that as a ship sailed away, its hull disappeared first before the sail, as if being enveloped by the water, indicating the Earth's curvature.

Eratosthenes (c. 276-194 BCE)

• A Greek mathematician, geographer, and astronomer who calculated the Earth's circumference with remarkable accuracy.
• Eratosthenes used the angles of shadows in Alexandria and Syene at noon on the summer solstice and applied geometry to estimate Earth's size.
• Eratosthenes was also the chief librarian at the Library of Alexandria, and developed a system for mapping the known world.
• Eratosthenes work laid the foundation for later advancements in geography and astronomy.
• In the Southern Egyptian city of Syene (now Aswan), at noon on the summer solstice, the Sun was directly overhead.
• In the Northern Egyptian city of Alexandria, about 800 km north of Syene, vertical objects cast a shadow.
• Eratosthenes measured the angle of the shadow cast by a vertical stick (a gnomon) in Alexandria and found it to be about 7.2 degrees.
• The 7.2-degree angle corresponds to 1/50th of a full circle (360 degrees).
• The distance between Syene and Alexandria was approximately 800 km.
• Since 7.2° represents 1/50 of Earth's circumference, multiplying 800 km by 50 gives 40,000 km, which is very close to modern measurements.