Ancient Greek Cosmology: Understanding the Universe

Early Greek Conceptions: The Ionian School

Some Greek thinkers made the first attempts to conceive the world as the result of natural processes, rather than an incomprehensible work of the gods. This was notably the case for the philosophers of the Ionian school, which flourished around the 6th century BC.

According to their views, the universe was initially in a state of primordial unity, where everything was mixed. From this unity, pairs of opposites arose, interacting with each other. These interactions led to the formation of heavenly bodies on one hand, and the Earth with its plants and animals on the other.

The Ionian philosophers conceived the Earth as a flat disk floating in the center of the celestial sphere.

The Spherical Earth: Pythagoras and Beyond

However, by the 5th century BC, the Greeks had realized through several indications that the Earth is round. To our knowledge, the legendary Pythagoras was probably the first to affirm this, coming to this conclusion based on observed facts. Yet, his arguments were also metaphysical: the Earth had to be spherical because, supposedly, the sphere is the most perfect geometric body.

Not all Greek philosophers accepted that the Earth, apparently so strong and solid, could have its own motion.

Philosophical Models of the Cosmos

Plato's Ideal Universe (427-347 BC)

Plato described the created universe as round, with stars moving circularly. For him, the sphere was the most perfect body, and circular motion was the most perfect form of movement.

Aristotle's Geocentric Model (384-322 BC)

Aristotle explicitly stated that the Earth is spherical and remains stationary at the center of the universe, with the heavens and all its stars revolving around it.

Measuring the Cosmos: Ancient Greek Astronomy

Greek astronomers were pioneers in seeking practical ways to measure the dimensions of the world they inhabited, moving beyond mere speculation or myths.

Eratosthenes and Earth's Circumference (2nd Century BC)

Eratosthenes, who lived in Alexandria in the 2nd century BC, successfully measured the Earth's circumference. He observed that on the day of the summer solstice, shadows fell vertically in Syene (modern Aswan), while in Alexandria, they formed a measurable angle with the vertical.

By measuring this minimum angle and the distance between Alexandria and Syene, Eratosthenes calculated the Earth's circumference to be 252,000 stadia. In modern units, taking the most probable value of the stadium, this equates to approximately 39,690 km – remarkably close to the actual value.

Later Greek Astronomy: Ptolemy's Influence

In summary, by the 2nd century BC, the Greeks had an excellent understanding of the size of the Earth and Moon, and the distance separating them. However, they placed the Sun much closer than its actual distance.

The last major Greek astronomer of antiquity was Ptolemy. He accepted and further developed the idea that the Earth is the center of the universe and that all celestial bodies revolve around it, a model that would dominate Western thought for over a millennium.