$$\require{cancel}$$

# 2.18: Greek Astronomy

Ancient cultures like the Babylonians and the Egyptians could recognize patterns in the sky. They could predict astronomical events. They had accurate calendars. However, they made no progress in answering fundamental questions about their universe. How far away are the planets, the Sun, and the Moon? Why do they shine? What makes them move? Progress was made by a remarkable group of thinkers who lived in the 3rd to 6th centuries B.C. on what is now the coast of Greece and Turkey. This era marks the birth of science.

The breakthrough was made by a set of philosophers who developed the basis of the scientific method. They applied logic and rigorous thought to many areas of human activity. Astronomy was just one example. They also developed new tools in mathematics to carry them forward. When Plato founded the world's first university in an olive grove outside Athens, he elevated mathematics to high intellectual status. The inscription above the main entrance read "Let Only Geometers Enter." The Greeks used geometry to move beyond the impression of the stars and planets as points of light on a fixed backdrop. They reached out to the third dimension and formed startling ideas about the true size of the universe.

Geometry of a total Solar eclipse. Click here for original source URL.

Total Solar eclipse 1999 in France. Click here for original source URL.

In 584 B.C. two Greek tribes were engaged in a bloody battle on the coast of Asia Minor. The poet Hesiod recorded the scene with vivid descriptions of burnished shields and flashing swords and carnage. Suddenly, the sky darkened and the air chilled with a total eclipse of the Sun. The soldiers wandered dazed and confused from the battlefield, believing they had witnessed an omen from the gods. Not far away, a man called Thales was not at all surprised. He had used Egyptian eclipse records to predict the exact date of the eclipse.

Thales was a statesman, geometer and astronomer who lived in Miletus in what is now Turkey from 634 to 556 B.C. He is the subject of perhaps the first anecdote about absent-minded scientists. A story is told of a servant girl who saw him fall into a well and chastised him for being so preoccupied with the heavens that he failed to notice what was under his feet. Thales believed that everything in the material world derived from water. While this might seem naive, it is an important step in the development of science to suppose that all things have a source and to suppose that the source of all things is one thing. This is a basic scientific idea - the diversity of the natural world conceals an underlying simplicity.

The ideas of Pythagoras are perfect examples of the union between mathematics and cosmology. He lived from 583 to 510 B.C. Pythagoras left no writings, so we know of his work only through the writing of his followers. By observing the phases of the Moon, he realized that the Moon is a sphere and proposed the then-unusual idea that Earth is a sphere, too. Pythagoras placed the spherical Earth at the center of the universe. He developed many ideas in mathematics, for example the famous "Pythagorean theorem" of right-angled triangles. Since a sphere is the most symmetric and perfect shape, it was natural to Pythagoras that it would describe the Earth and the orbits of the Sun, the planets, and the stars. Pythagoras established a school in southern Italy, and some of his students proposed the idea that the Earth, the planets, the Sun, and the stars all move around some distant, central "fire." Even though some of these early ideas are incorrect, we can recognize the bold and adventurous thinking of these early scientists.

Geometry of a Lunar eclipse. Click here for original source URL.

Lunar eclipse. Click here for original source URL.

In the 5th century B.C., Anaxagoras deduced the true causes of eclipses. He noticed the curved shadow of the Earth on the moon during a lunar eclipse and realized that this observation supports the idea that Earth is round. A sphere is the only solid shape that casts a circular shadow regardless of the direction of illumination. He studied a meteorite that had fallen out of the sky in 467 B.C., and calculated that the Sun is an incandescent "stone" even larger than Greece. This idea got him in trouble when he was charged with impiety and banished for teaching heretical ideas.