30 Facts About Ptolemy

Claudius Ptolemy was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine, Islamic, and Western European science.

The 14th-century astronomer Theodore Meliteniotes wrote that Ptolemy's birthplace was Ptolemais Hermiou, a Greek city in the Thebaid region of Egypt.

Ptolemy had a Latin name, Claudius, which is generally taken to imply he was a Roman citizen.

Ptolemy was familiar with Greek philosophers and used Babylonian observations and Babylonian lunar theory.

Ptolemy's Greek name, Ptolemaeus, is an ancient Greek personal name.

Ptolemy wrote in Koine Greek, and can be shown to have used Babylonian astronomical data.

Ptolemy might have been a Roman citizen, but was ethnically either a Greek or at least a Hellenized Egyptian.

Ptolemy presented his astronomical models alongside convenient tables, which could be used to compute the future or past position of the planets.

Objections were raised by Bernard Goldstein, who questioned Newton's findings and suggested that he had misunderstood the secondary literature, while noting that issues with the accuracy of Ptolemy's observations had long been known.

Ptolemy goes beyond the mathematical models of the Almagest to present a physical realization of the universe as a set of nested spheres, in which he used the epicycles of his planetary model to compute the dimensions of the universe.

Ptolemy estimated the Sun was at an average distance of Earth radii, while the radius of the sphere of the fixed stars was times the radius of the Earth.

The Analemma is a short treatise where Ptolemy provides a method for specifying the location of the Sun in three pairs of locally oriented coordinate arcs as a function of the declination of the Sun, the terrestrial latitude, and the hour.

The key to the approach is to represent the solid configuration in a plane diagram that Ptolemy calls the analemma.

The text, which comes from an eighth-century manuscript which contains Ptolemy's Analemma, was identified on the basis of both its content and linguistic analysis as being by Ptolemy.

Ptolemy is though to have produced his Table of Noteworthy Cities as an aid to his astronomical tables.

Ptolemy relied on previous work by an earlier geographer, Marinus of Tyre, as well as on gazetteers of the Roman and ancient Persian Empire.

Ptolemy acknowledged ancient astronomer Hipparchus for having provided the elevation of the north celestial pole for a few cities.

Latitude was measured from the equator, as it is today, but Ptolemy preferred to express it as climata, the length of the longest day rather than degrees of arc: The length of the midsummer day increases from 12h to 24h as one goes from the equator to the polar circle.

One of the places Ptolemy noted specific coordinates for was the now-lost stone tower which marked the midpoint on the ancient Silk Road, and which scholars have been trying to locate ever since.

Much of the content of the Tetrabiblos was collected from earlier sources; Ptolemy's achievement was to order his material in a systematic way, showing how the subject could, in his view, be rationalized.

Ptolemy dismisses other astrological practices, such as considering the numerological significance of names, that he believed to be without sound basis, and leaves out popular topics, such as electional astrology and medical astrology, for similar reasons.

The identity and date of the actual author of the work, referred to now as Pseudo-Ptolemy, remains the subject of conjecture.

Ptolemy wrote a work entitled Harmonikon, known as the Harmonics, on music theory and the mathematics behind musical scales in three books.

Ptolemy introduces the harmonic canon or monochord, which is an experimental musical apparatus that he used to measure relative pitches, and used to describe to his readers how to demonstrate the relations discussed in the following chapters for themselves.

Ptolemy offered explanations for many phenomena concerning illumination and colour, size, shape, movement, and binocular vision.

Ptolemy divided illusions into those caused by physical or optical factors and those caused by judgmental factors.

Ptolemy offered an obscure explanation of the Sun or Moon illusion based on the difficulty of looking upwards.

However, according to Mark Smith, Ptolemy's table was based in part on real experiments.

Ptolemy argues that, to arrive at the truth, one should use both reason and sense perception in ways that complement each other.

Elsewhere, Ptolemy affirms the supremacy of mathematical knowledge over other forms of knowledge.