Mathematics was More Important than Religion in Ancient Greek Astronomy

Ancient Greek Astronomy, which also encompasses Roman astronomy, has its earliest references in the writings of Homer, and detailed a flat-earth viewpoint where the stars would rise and fall from the world-spanning ocean at different parts of the year. It was used for astrology and fortune telling, and derived a great deal from the Babylonian Astronomy that came before it. The Greeks invented the "order of magnitude" system for determining the brightness of stars in 135 BC.

The Role of the Planets

Our very term for celestial bodies, "planets" comes from the Greek word "planetes", meaning "wanderer", and the first ideas of a universe centered on something other than the Earth stem from Greek thought. The five naked-eye visible planets were plotted against a combined solar and lunar calendar by the Greeks, and it was the Pythagoreans who eventually figured out that the Evening Star and the Morning Star were, in fact, the same wandering planet Venus.

The Motion of the Planets

The science of attempting to determine why the planets move the way they did dates back to ancient Greek Astronomy, and while Eudoxus' model (described above) was the root, it eventually branched out with more spheres, spheres that were slightly off center to one another, and planets that were on rotating circles on spheres that rotated as well – if this is making your head spin, you're not alone. This attempt to mathematically describe how the planets moved was one of the brilliant attempts of the classical age, and is a good example of very smart people putting a lot of work into a detailed science, while missing one critical element – that the planets, including the Earth, rotate around the Sun.

The Birth of a Sun Centered Model

It was in ancient Greek Astronomy that a sun centered cosmos was born. Aristarchus proposed a sun-centric model of the cosmos in the 3rd century BC, which was not well received. It wasn't until Copernicus, nearly 2,000 years later, that this idea gained acceptance. Aristarchus also took the lengths of shadows of monuments to derive the curvature of the Earth – measuring the shadow length at noon on the summer solstice for a monument at Alexandria and another at Thebes. From the differences in shadow lengths, and a little bit of experimentation, he was able to derive the degree of curvature of the Earth. By plugging in the distance between the two monuments, he was able to give a rough estimate of the size of the Earth. (Since his initial measurement for the distance between the two monuments was in error, his calculated size of the Earth was too small by almost 40%). He also used these same procedures to estimate the distance between the Sun and Earth, and the Moon and Earth in earth radii.

Ancient Greek Astronomy and Greek Life

Unlike the Babylonians and Egyptians, ancient Greek Astronomy was mathematical, rather than religious in nature. This view of Astronomy was profoundly important in the differentiation between Astronomy and astrology. Greek astronomical thinking formed the foundation of much scientific and mathematical thought that laid the foundations for the world we live in today.