this page honors Claudius Ptolemaeus, known today as Ptolemy (ca. A.D. 100-175)

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Cygnus (The Swan) is one of 48 constellations contained in the Almagest's star chart. According to Ptolemy, there are 17 stars that form the pattern, starting with "The star on the beak" (in the modern designation, β Cyg), and finishing up with "The one to the rear" (ο² Cyg).

Ptolemy was a man with a Roman name who lived and worked in Alexandria, Egypt, and who wrote prolifically about astronomy, geography, music, optics, and other subjects in the Greek language. Not much is known about his life, and even the dates of his birth and death can only be estimated.

By our standards, Ptolemy had access to a very small toolkit.

He had to make do without algebra and calculus, and the invention of the telescope was more than a millennium in the future. He could use geometry, but it was so limited that he had to make use of a complicated system of circles — the epicyclic hypothesis — to define the orbits of the planets in the solar system.

However, he did have access to several centuries' worth of observational data.

The data that he used was collected by Babylonian and ancient Greek astronomers, including data collected by Hipparchus that we no longer have available from the original sources. At the time, the library in Alexandria was famous for the high quality of its collection of documents, and he certainly made use of these.

Ptolemy also made use of the astrolabe while making his own observations.

His writings include detailed instructions, with diagrams, on building and using such a device as well as a parallactic instrument that made use of sighting plates and rods.

In spite of these limitations, Ptolemy's collected works contain detailed observation data tables, geometrical proofs of his theories, the earliest surviving table of a trigonometric function, and a star chart that contains the coordinates, color designations, and magnitude measurements for more than a thousand stars in 48 constellations. His goal was to create an astronomy collection that was "directed only toward scientific usefulness, and not towards ostentation."

Ptolemy believed that the motions of the heavenly bodies could be explained in mathematical terms.

The original name for his collection was The Mathematical Collection. It contains 13 separate books, became popularly known as the Almagest ("The Greatest", from a hybrid of Greek and Latin) after it was published in approximately A.D. 150. By the early fourth century it had become the standard textbook on astronomy, and it wasn't superseded until the sixteenth century.

To read more:

Ptolemy's Almagest, translated and annotated by G.J. Toomer. Springer-Verlag, ©1984.