Ptolemy Today

Around 150 CE, the Greco-Egyptian astronomer Claudius Ptolemy wrote the most influential astronomy textbook ever produced. For nearly fifteen centuries his Almagest was the standard work on the motions of the heavens. It is a technical masterpiece — and also, for most modern readers, almost unreadable.

That is a shame, because once you strip away the ancient notation and the dense geometry, the Almagest is one of the best introductions to basic astronomy ever written. It starts from what you can see in the night sky and builds up, step by step, to a complete working model of how the Sun, Moon, and planets move. No other text takes a beginner through that journey so carefully.

This project is an attempt to recover the Almagest as a living text. The plan is to walk through its key arguments in accessible modern prose, illustrated with interactive three-dimensional diagrams that show what Ptolemy is describing. The mathematics is mostly set aside; the concepts, the reasoning, and the geometry are foregrounded.

In each chapter, the main body text is a rewritten version of what Ptolemy actually wrote. Any additional explanation — historical context, modern connections, or corrections — is placed in tinted boxes or collapsible sections like this one, so you can always tell what is Ptolemy and what is commentary.

It is a work in progress. New chapters will be added as they are written.