Astronomy in Ancient Greece

In the early days of ancient Greece, astronomy, mythology, and astrology were all connected. Priests would interpret signs in the skies as messages from the gods. Early myths about heroes and gods were often translated into constellations in the night sky.

This was a common phenomenon throughout early civilizations. Some peoples, like the Babylonians, would study planets, stars, and constellations in order to create extensive charts for predicting the future, or astrology. Others, like the Egyptians, learned that the arrival or departure of certain planets or stars led to the rise and fall of the Nile River, which was so crucial to their agriculture.

The Greeks not only learned from these civilizations, but also developed their own concept of astronomy—based in part on philosophy and in part on observations by the naked eye (since the invention of the telescope would not come for many years). Based on such observations, the early Greek poet Homer wrote of celestial objects and events, indicating solar eclipses and noting constellations such as Orion, Bootes, and Ursa Major.

The poet Hesiod mentioned the star Arc- turus in the constellation Bootes. The Greeks viewed astronomy as a part of mathematics in which parts of the world were associated with each other in a geometrical model. Pythagoras placed astronomy, arithmetic, geometry, and music as subdivisions of math.

Most of the Greek astronomy that has come down to us is based upon the work of Aristotle. He often described earlier ideas, usually to argue against them, but in doing so he gave us representations of the earlier concepts. Pythagoras (571-497), a Greek philosopher from Samos, argued that the Earth was spherical. His teachings were never written down, though, and he appears to have been a mystic.

He argued that math was the key to enlightenment, and one of his lasting contributions in this realm was the Pythagorean theorem, which postulated that the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides. His students furthered his ideas of applying math by showing that based on how the shadow of the Earth appeared on the Moon during eclipses, the Earth had to be a sphere.

Other Greek philosophers did not share this view. Some argued that the Earth was floating in water, while others believed that it was flat like a lid covering the world below it. But all of these ideas argued that the Earth was center of the universe, a geocentric view. The philosophy behind this came from observations of the cosmos, and in particular the solar system, which supposedly showed that everything revolved around a stationary Earth.

The philosophers Plato and Aristotle believed that the other planets and stars revolved around the Earth in concentric circles; again, Plato continued the idea of a mystical approach to the cosmos not governed by laws. For example, Mars did not move in any discernable pattern, so it must not subscribe to any form of natural law.

Aristotle discussed the different positions of the polar star based upon one’s position in Egypt, Cyprus, and Greece, and when one moved north, it changed; hence the idea of latitude and a spherical Earth. Some of Plato’s students argued against their teacher by trying to apply mathematics to understanding and predicting the motions of the heavens. Nevertheless, the idea of concentric, circular patterns of planetary movement was appealing and continued to be espoused.

With the assumption that the Earth was spherical being accepted, attempts were made to determine its circumference. Aristotle gave it at about 64,000 kilometers by comparing the position of the polar star in Egypt and Greece. Eratosthenes (276-195) used geometry and shadows cast at two different latitude sites (Alexandria and Syene) at the same time to come up with 44,000 kilometers, a figure not rivaled until the modern age (and within 10 percent of the actual distance, 40,074 kilometers).

The idea of a geocentric view of the cosmos was not universally accepted, although it was the predominant one. It did not explain changes in brightness or motion of other planets seen in the sky. Aristarchus of Samos (310-230) postulated that the Sun was the center of the universe, a heliocentric view. Although his work is now lost, another ancient mathematician, Archimedes, quoted him saying that the Sun did not move and that the Earth revolved around it in a circle.

His view did not catch on for a long time, however. Furthermore, his idea assumed that planets revolved around the Sun in a circular fashion, which they do not; they move elliptically, which again went against the ideas of concentric circles of early philosophers. Opposed to Aristarchus’s view was Hipparchus of Nicea (190-120), who became a respected astronomer and outshined his predecessors by calculating the lengths of the lunar month and solar year accurately.

He produced a precise latitudinal and longitudinal star chart and compared it to an earlier star chart, discovering the Equinoctial Precession, where the stars shifted about 1 degree per century, rotating as if they were cones. The Earth’s precession takes about 26,000 years, shifting 1 degree every seventy-two years, where the positions of the stars change in their equatorial coordinates and ecliptic longitudes so they appear in different places over time. For instance, since the time of Hipparchus, the change has been 30 degrees.

The completion of the mathematical model was done by Apollonius of Perga in 200, and it was perfected later by Hipparchus, who argued that the circles of the planetary movement were based on eccentric circles, but Earth was not in the exact center. In this way, the planets and stars moved faster as they were near the Earth and slower when farther away, hence the observation of the changes in brightness and speed.

The ultimate conclusion of Greek astronomy occurred under Ptolemy and his Almagest in the second century CE, which contained ideas from Greek and Babylonian philosophies. He included a catalog of more than 1,000 known stars, as well as tables to show how their movements could be calculated. Although Ptolemy produced this astronomical work, he probably did it in line and in conjunction with astrology, for he also produced the Tetrabiblos, a work on astrology using astronomical materials.

While the Earth-centered philosophy was dominant, it required extensive explanations of all the seen phenomena, such as the Moon, the stars, and the seasons. Ptolemy, building upon the work of other Greek astronomers, had to derive complicated mathematical ideas and formulas to account for the Sun revolving around the Earth and other planets moving in similar ways.

While the study of astronomy and celestial objects was often described in mathematical terms, astronomy was also linked to astrology. In fact, the two terms were used interchangeably. Like other societies, the Greeks believed that celestial objects could influence an individual’s life and they also thought that stars, planets, and other phenomena had special powers.

These objects, especially planets, exerted influences such as destinies, auspicious moments, and divine intentions. The Greeks viewed these forces as having a direct influence on their lives, and they believed that the gods controlled these forces through the heavens. Astrology was explained through the religious nature of Greek society.

This aspect involved both public cults and private beliefs. Ancient authors described peculiarities involving individuals and their superstitions. The philosopher Theophrastus, writing during the time of Alexander the Great, included in his work Characters a passage on the Superstitious Man, who would wash his hands in a fountain, put a bit of laurel leaf in his mouth, and sprinkle himself with temple water.

He would not continue his walk until another traveler passed him, or if a weasel ran across his path, or if none came, he would throw three stones across the path before continuing. He would not walk on a tombstone or come near a dead body. These acts and beliefs were common in the Greek world and showed how forces outside one’s control interacted with people, for good and ill.

Astrology and astronomy sought to explain how celestial objects and phenomena influenced society. This influence could be at the level of the state—for example, an eclipse could foretell a bad or good event (such as what Thucydides experienced during the Sicilian Campaign)—or it could occur at the individual level, when a person’s day and time could be compared with the stars and planets and allow for future planning. Science and religion were connected with each other and allowed for an explanation of events and the heavens.