Early Astronomy in the University of Michigan Collections
Early Greek Astronomy
The motion of the wandering stars remains a problem throughout antiquity and ephemerides, tables listing the positions of the heavenly bodies at intervals of one day were compiled until late Antiquity.
Greek astronomy, as in many other civilizations, developed in phases.
Star gazing and its utility are already mentioned in the first two authors of Greek poetry: Homer and Hesiod. In particular, in his Works and Days (seventh century BCE) Hesiod already discusses farming seasons in terms of the risings and settings of certain constellations (e.g., Pleiades, Arcturus, and Sirius). In the sixth and fifth century BCE Presocratic philosophers, though not primarily interested in studying the sky, all shared an interest in astronomy since their main focus was the study of Nature in general. While Thales studied solar eclipses, Anaximander is credited with the use of a gnomon to determine occurrences of solstices and equinoxes. Famously, Anaxagoras (500-428 BCE) explained the Moon’s phases and eclipses, and in 431 BCE was sent into exile with the accusation of impiety because he asserted that the Sun was an incandescent piece of stone (while for the Greeks it was a god, Helios).
In the second half of the fifth century astronomers like Meton and Euctemon dealt mostly with calendrical matters, an important problem in Greece since lunar calendars were used (often changing from city to city) and these needed to be synchronized with the solar year, which regulates and determines the seasons. Further, by the fourth century BCE the Greeks knew that the earth was spherical.
Plato (428-348 BCE) did not have any specific astronomical interests, and the cosmological descriptions in the Timaeus and in the myth of Er in the tenth book of the Republic are mostly “myths”, even if they rely on ideas of the celestial sphere and of the geometrical shape of the cosmos. Indeed, in Plato’s Academy mathematics (which largely corresponds to modern geometry) had a preeminent place. This was of great importance for later developments because Greek astronomy consisted more and more of geometry, in order to develop geometrical models aimed at predicting the movement of celestial bodies. In fact, as the story goes, Plato had challenged future astronomers to “save the phenomena”: this referred to the fact that at that time the Greeks already recognized that, unlike the fixed stars, the planets (which the Greeks called “wandering stars”) do not have a regular path and thus were at odds with the perfectly regular order of the universe. Thus, “saving the phenomena” became finding a theory that, while geometrically correct, could account for and predict the supposedly irregular paths of planetary motions.
Eudoxus (ca. 390-337 BCE), a pupil of Plato, was the first to propose a solution. He devised a series of 27 concentric spheres rotating around the earth; each planet’s path was explained with four connected spheres whose combined movement (resulting in a curve called hippopede) would reproduce the planet’s apparently erratic motion. This was probably just a mathematical model in the sense that Eudoxus probably did not think that these spheres really existed in the Universe but rather only that they served to explain observations mathematically. We do not have the original work of Eudoxus, but fragments are preserved by Aristotle (384-322 BCE), who further developed Eudoxus’ theory by adding more spheres (for a total of 56 spheres!). In Aristotle’ cosmos there is a series of concentric spheres and at the end an “Immovable Mover” who sets all these spheres in motion; at the center of the universe there is the Earth, at rest. Unlike Eudoxus, Aristotle considered these spheres to be real, and his theory impacted astronomy for centuries, as Aristotle was considered the authority in many scientific fields through the Middle Ages.