Early Astronomy in the University of Michigan Collections

The use of spheres to explain the motion of stars and planets was indeed the key for the development of a more scientific astronomy. The Greeks developed a model of the cosmos as a sphere and introduced the concept of celestial sphere. This is fully developed in the Hellenistic period with Euclid’s Phenomena (ca. 320 BCE), which discusses both the celestial sphere and all the main circles drawn on its surface (celestial equator, tropics, circumpolar circles, ecliptic).

In the Hellenistic period a series of astronomical texts were composed that dealt with the concept of the celestial sphere and spherical astronomy to tackle some astronomical problems (e.g., the risings and setting times of constellations), such as Autolycus’ On the Moving Sphere and On Risings and Settings (ca. 320 BCE). The observations by Aristyllus and Timocharis carried out at Alexandria in ca. 290 BCE, which have reached us through Ptolemy, are the oldest observations we have in Greek astronomy.

Aristarchus of Samos (ca. 310-230 BCE) is one of the most prominent astronomers of the Hellenistic period. Although Greek astronomy remained fundamentally geocentric (due also to the influence of Aristotelian physics), in his Sand Reckoner Archimedes attributes to Aristarchus the idea that phenomena could be saved also by assuming the Sun to be the center of the cosmos. Little of Aristarchus’ theory has reached us directly, as the only work we have by him is a treatise in which, through purely geometrical methods, he attempts the measurement of the relative distances of the Sun, Moon, and Earth (On the Sizes and Distances of the Sun and Moon).

The other great Hellenistic astronomer is Hipparchus (early second century BCE- d. after 127 BCE); he was a fundamental figure in the development of astronomy. Unfortunately, nothing of his work has been preserved with the exception of a polemical commentary against Aratus’ Phaenomena. Still, mainly through Ptolemy, we know that Hipparchus discovered the precession of the equinoxes by comparing his own observations to those of Timocharis and Aristyllus. He also worked on models for the motions of the Sun and of the Moon using two equivalent geometrical models, the eccentric-circle model (or eccenter model) and the concentric-and-epicycle (also known as deferent-and-epicycle model) model, which might have been devised by Apollonius of Perga (ca. 240-180 BCE). Hipparchus was probably able to improve so much on the previous theories because he had access to Babylonian records, which allowed him to produce more sophisticated models by having a database of observations covering a much longer period of time. From Babylonian astronomy Hipparchus also adopted the concepts of degree and zodiac sign (even if the latter was already known to Euclid and Autolycus) and he was probably the first to use celestial coordinates to indicate the position of a star in the celestial sphere. Around the same time as Hipparchus, Hypsicles in On Ascensions utilized Babylonian arithmetical methods to calculate the rising times of the zodiac signs.

The Hellenistic period, in parallel to mathematical (or geometrical) astronomy, also saw a popularization of astronomy. The study of the fixed stars was made easy and fashionable by the poem of Aratus, the Phaenomena (ca. 275 BCE); a bit later, Eratosthenes (ca. 276-196 BCE) also prepared a list of constellations adding myths to explain how the constellations originated (the Catasterismi). The interest in constellations and their risings was also important for calendrical matters, for which a specific genre existed: the star calendars or parapegmata (plural; singular is parapegma), which showed which stars were rising across the whole year and provided meteorological predictions connected to them. We have both epigraphic remnants displayed in public (e.g., the parapegma of Miletus) as well as textual examples (the parapegma at the end of Geminus’ Introduction). Astrology also started to become popular in the Graeco-Roman world at the end of the Hellenistic period. This astrology consisted of a combination of Greek, Babylonian, and Egyptian astronomical and astrological ideas, and focused on personal horoscopes, as shown by the large number of papyri of horoscopes that have reached us. Yet, the popularity of astrology gave additional impulse to the development of mathematical astronomy in order to better track the movement of the planets and stars.

Ptolemy and Later Greek Astronomy

Babylonian and Greek Astronomy