## Greek Astronomy

The modern scientific view of the universe has its roots in ancient Greek tradition of philosophy.

Pythagoras of Samos, ca. 532 BC: Connected religion with maths. Created the abstract idea of mathematics. He believed that “all things are numbers”, and that arithmetic and the physical world are intimately related. He first demonstrated the fundamental relationship between numbers and nature.

Initially, the goal is to describe the motion of the celestial objects:

• Stars
• Sun and moon
• Planets = “Wandering stars”. These posed the greatest challenge to the ancient world before Copernicus.

Plato, ca. 350 BC: suggested that to understand the motion of the heavens, one must set initial hypotheses:

• Stars revolve around a fixed central Earth.
• Heavens must follow the “purest” possible motion = circular motion at constant speed.

These assumptions seem to be the most logical to explain the motion of the stars, since their positions remain apparently constant relative to each other. Indeed, the assumptions do explain the motion of the stars as seen from Earth.

Basic geocentric model of the universe, showing the celestial sphere rotating with the Earth in the center.

Celestial sphere: Sphere where stars are attached. Such sphere has north and south celestial poles.

The problem with wandering stars: these have a retrograde motion pattern in the sky which is not possible to explain with the above model. Watch the next video to understand the retrograde motion.

Or look at this pic:

To explain this motion, Hipparcus (ca. 150 BC) proposed a system of circles. He placed planets on small rotating epicycles that in turn moved on a larger deferent. This was refined by Ptolemy (ca. AD 100) by adding equants, and these allowed the angular momentum of the planet to be constant (see video below). He also moved Earth away of its deferent center. Predictions of the Ptolemaic model agreed more closely with the motion of the stars than any other previous scheme, but this model also grew more and more complex as little corrections were added when needed.

Ptolemaic model. With this, the planet moves in retrograde motion with respect to Earth.

This video may help a bit to understand (skip to the final part):

The Ptolemaic model became almost universally accepted, and would be kept until Copernicus heliocentric theory took over.

Resources:

History of Western Philosophy, by Bertrand Russell: Plenty of information about ancient Greek philosophy relatively accessible to everyone.