Early Astronomy in the University of Michigan Collections
Ptolemy and Later Greek Astronomy
The astronomical tradition of the Hellenistic and Roman period was collected and systematized by Claudius Ptolemaeus or Ptolemy (ca. 105-175 CE). He is the most prominent astronomer of antiquity and the one whose works have reached us most extensively. His Almagest (the name derives from the Arabic title al-Majisṭī, which in its turn derives from the Greek Megistē Syntaxis, “Greatest Collection”, itself a corruption of the original title, Mathēmatikē Syntaxis, “Mathematical Collection”) was published around 150 CE and became the foundation of western astronomy until Kepler. Ptolemy reorganized and reworked past theories, often improving on them through his own observations and calculations. In fact, Ptolemy could benefit from the mathematical work done by Menelaus in the 1st century CE (Ptolemy mentions two of his observations made in 98 CE). In his Spherics Menelaus studied the geometry of spherical triangles; even if his original work is lost (we only have translations in Arabic), it probably marked the beginning of spherical trigonometry, a new discipline which finally made it possible to carry out precise calculations on the celestial sphere. In particular, the so-called “Menelaus’ Theorem” was very much used by Ptolemy and by subsequent astronomers (many Islamic astronomers wrote works on it).
Ptolemy also organized the first scientific catalogue of stars (with ecliptic coordinates, for the year 137 CE), which is the basis of modern star catalogues; his model for solar and planetary motions (deferent-and-epicycle model with equant point) was so precise at predicting planetary and solar motions that later astronomers up to the time of Kepler mostly just modified it rather than proposing major changes. This included Copernicus who adapted many parts of Ptolemy’s theories and models to a heliocentric model. Ptolemy also wrote about astrology, in a work known as the Tetrabiblios, because during his time astrology and astronomy were closely connected and not considered incompatible as in modern science.
After Ptolemy Greek astronomy greatly slowed its progress; the works of later astronomers mostly consist of commentaries (for example, those of Pappus and Theon on Ptolemy’s works, dating to the fourth century CE) and collections of past achievements, mostly for school uses. The teaching was organized through the so-called Little Astronomy (a collection of the following works, considered preparatory to the studying of Ptolemy’s Almagest: Theodosius, Spherics; Autolycus, On the Moving Sphere; Euclid, Optics; Euclid, Phenomena; Theodosius, On Geographic Places; Theodosius, On Days and Nights; Aristarchus, The Sizes and Distances of the Sun and Moon; Autolycus, On Risings and Settings; Hypsicles, On Ascensions; Euclid, Catoptrics; Euclid, Data) and then the Almagest (or Syntaxis). In fact, the immense success of Ptolemy’s Syntaxis also caused the works of the previous astronomers such as Hipparchus to be forgotten. The other texts that survived and reached us directly were the easier texts such as Aratus’ Phaenomena or introductions to astronomy such as the ones written by Geminus (first century BCE) and Cleomedes (ca. 200 CE), aimed at amateurish readers or beginner students rather than at professionals.