29 Facts About Ptolemy

Ptolemy lived in or around the city of Alexandria, in the Roman province of Egypt under Roman rule, had a Latin name, which is generally taken to imply he was a Roman citizen, cited Greek philosophers, and used Babylonian observations and Babylonian lunar theory.

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

Several historians have made the deduction that this indicates that Ptolemy would have been a Roman citizen.

Ptolemy wrote in ancient Greek and can be shown to have utilized Babylonian astronomical data.

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

Ptolemy, following Hipparchus, derived each of his geometrical models for the Sun, Moon, and the planets from selected astronomical observations done in the spanning of more than 800 years; however, many astronomers have for centuries suspected that some of his models' parameters were adopted independently of observations.

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

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 1,210 Earth radii, while the radius of the sphere of the fixed stars was 20,000 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 orientated 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.

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.

Ptolemy's oikoumene spanned 180 degrees of longitude from the Blessed Islands in the Atlantic Ocean to the middle of China, and about 80 degrees of latitude from Shetland to anti-Meroe ; Ptolemy was well aware that he knew about only a quarter of the globe, and an erroneous extension of China southward suggests his sources did not reach all the way to the Pacific Ocean.

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

Ptolemy introduces the harmonic canon, an experimental apparatus that would be used for the demonstrations in the next chapters, then proceeds to discuss Pythagorean tuning.

Pythagoreans believed that the mathematics of music should be based on the specific ratio of 3:2, whereas Ptolemy merely believed that it should just generally involve tetrachords and octaves.

Ptolemy presented his own divisions of the tetrachord and the octave, which he derived with the help of a monochord.

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.