Claudius Ptolemy (about 100-170) of Alexandria is the inventor of the Ptolemaic system, a theory of the heavens that said the stars, sun, moon and planets went round the earth. The earth did not move, it did not even turn. In the early 1600s this was replaced by the theory of Copernicus and Kepler that held that the earth and everything else went round the sun.
Ptolemy also wrote about music, astrology, optics and geography. He was among the first to apply trigonometry to science.
He wrote about his theory of the heavens in the “Almagest” (150) and other works. With it you can get the position of the sun, moon and planets on any given day, past, present or future.
Ptolemy built his theory on 25 years of his own observations and the work of Hipparchus — and probably the work of others (now lost).
The Almagest is a work of genius and beauty that stood for over a thousand years, but it is hardly perfect:
- We now know some of his observations were made up.
- It contains arithmetic errors that just happen to let his proofs come out right.
- It was based on Aristotle’s physics, some of which was easy to prove wrong if anyone took the trouble to check it out against the real world. Someone finally did: Galileo.
Ptolemy takes Aristotle’s physics as a given and then comes up with a theory that fits both Aristotle and his observations.
The root trouble with his theory is not what you think – where he put the earth – but his use of circles.
According to Aristotle heavenly bodies were made up of something called quintessence. Quintessence, being perfect moved in perfect circles. Aristotle said that was the perfect motion.
And so Ptolemy manfully stuck to circles. But to get his circles to match his observations, he needed circles within circles – the dreaded epicycles.
Planets move in stretched-out circles called ellipses, as Kepler later found out. It is not that Ptolemy could not do ellipses – it was just the sort of thing he was good at. It was his physics that held him back.
Copernicus used circles and epicycles too, so he was not that much better. It was not till the work of Galileo, Kepler and Newton that Copernicus’ theory won the day. Galileo proved it true, Kepler made it usable and Newton provided the physics.
Astrology: Ptolemy believed that the movements of the heavens affect us. In his book “Tetrabiblios” he shows how in terms of Aristotle’s physics.
Geography: his book on geography was not known in the West till 1300. In it he gives the latitude and longitude of over 8000 places from Spain to China, making possible a detailed map of the world as it was known in Alexandria in his day. It is from Ptolemy that we get the idea of north being “up.”
Ptolemy knew the earth was round but thought it was smaller than it really is. That is why Columbus thought that he could easily get to Asia by sailing west across the ocean.