Ancient Greek Astronomy and Cosmology: A Foundation of Knowledge and Inquiry

Ancient Greek astronomy was built on earlier Babylonian and Egyptian systems, later influencing Islamic and early modern European scholars. The Greeks made significant strides in understanding celestial movements, the Earth’s shape, and cosmic structure, using mathematics and observation to ground their theories.

One of the strongest pieces of evidence of the shape of the Earth came through interpretation of a lunar eclipse. During an eclipse we see the shadow of the Earth on the moon. This looks like the image at the top. If the Earth were triangular, one would expect the shadow of the Earth to look like a triangle. Diagrams of solar eclipse, from Universal Geography, 1711. Geography and Maps Division

One of the other key pieces of evidence for a round Earth was the fact that at a distance one can see the top of a ship before seeing the rest of it. One can find this concept illustrated in a wide array of astronomical texts throughout history. M. Blundeuile his exercises. 1613. 1613. The Kraus Collection of Sir Francis Drake, Rare Book and Special Collections Division

This illustration from a translation of Aristarchus work shows how he approached measuring the size of the Earth based the size of the Earth’s shadow on the moon during a Lunar eclipse Geometric figure of Earth, Sun, and Moon calculated by Aristarchus to approximate real scale of the solar system Illus. in: De magnitudinibus, et distantiis solis, et lunae 1572, p. 30.

This illustration from the French edition of the 13th century encyclopedic work De proprietatibus rerum (The Properties of Things) shows Aristotle’s four elements. Fire and air are illustrated at the top of the image and water and earth on the bottom. . Le proprietaire en francoys. (The Properties in French). 1491. Rare Book and Special Collections Division

An illustration of an interpretation of the heavenly spheres. The round Earth is surrounded by spheres of water, air, and fire and from there the spheres of the moon, each of the planets, and the fixed stars. The Kraus Collection of Sir Francis Drake, Rare Book and Special Collections

Here you can see a medieval interpretation of the notion of the heavenly spheres. On the bottom is a depiction of the Earth, above that is fire (the lightest of the Aristotelian elements). Beyond that one finds band of the moon, followed by Mercury and Venus, then the sun, followed by Mars, Jupiter and Saturn, and beyond that the band of the fixed stars and beyond them the angels and heaven. The distinct colors and firm lines separating each band underscores the belief in these as layers that did not intersect.Buch der Natur (Book of Nature). 1481. Rosenwald Collection, Rare Book and Special Collections Division

The Spherical Earth and Evidence

Ancient Greek ideas about the heavens became known as the Ptolemaic model, which had Earth at the center of the universe. In this image you can see each of the planets represented as the personifications of the gods they are named after. Each deity is seen riding a chariot through the heavens. Harmonia Macrocosmica, 1708. Geography and Maps Division.

By the 5th century BCE, it was already accepted among Greek thinkers, such as Anaxagoras and Empedocles, that the Earth was spherical. They drew this conclusion from observing lunar eclipses, noting the Earth’s round shadow on the moon. Additionally, sailors’ observations that the tops of ships appeared first on the horizon provided practical evidence of Earth’s curvature.

Eratosthenes later built on these ideas in 240 BCE, using differences in shadow angles between two locations, Alexandria and Syene, to estimate the Earth’s circumference. While the exact accuracy of his measurement is debated due to uncertainties in ancient units, his result was remarkably close to modern figures.

Aristotle’s Elements and Cosmology

An example of one of the many tables found in the Almagest. Well over a thousand years after Ptolemy put these tables together, they were being, commented on and revised. Almagest. 1515. Rare Book and S

Aristotle developed a system where four primary elements earth, air, fire, and water explained the natural world. Each element had intrinsic qualities and weight, with the heaviest elements, earth and water, settling toward the center of the universe. In Aristotle’s model, the heavens were composed of a fifth element, aether (or quintessence), representing purity and eternal, unchanging motion.

The ancient Greeks also categorized celestial bodies into fixed and wandering stars (planets). The fixed stars moved together across the sky, while the wandering stars, including the sun, moon, and visible planets, followed distinct patterns, governed by an unseen “unmoved mover,” often identified with gods.

Ptolemy’s Contributions and Complex Models

Illustration of the orbit of Mercury from Di Sphera Mundi originally published by Johannes de Sacrobosco. 1230. In this model, Mercury moved on its epicycle, illustrated as the three small circles. Di Sphera Mundi (Sphere of the World). 1482. Rare Book and Special Collections Division.

Claudius Ptolemy, working in Alexandria in the 2nd century CE, built on previous Greek knowledge to create detailed mathematical models for predicting planetary motions. His work, Almagest, included complex systems of circles upon circles, called epicycles, to maintain the Greek ideal of perfect circular motion. These epicycles, along with deferents and equants, allowed Ptolemy’s model to predict planetary movements accurately, though at the cost of complexity.

Legacy and Influence

Illustrating just how extensive Ptolemy’s impact was on astronomy, more than a thousand years after his work it was still being translated and commented on. Here in the frontispiece to astronomer Regiomontanus’ Epytoma in Almagestum one can see Ptolemy and his renaissance translator and commenter Regiomontanus seated beneath an Armillary Sphere. Epytoma Ioannis de Monte Regio in Almagestum Ptolomei. 1496 Rare Book and Special Collections Division.

Greek astronomy’s focus on mathematical models and observation established a tradition that deeply influenced Islamic scholars, who preserved and expanded on the knowledge through translations and commentaries. These contributions laid the groundwork for European Renaissance astronomers like Copernicus, who eventually challenged the geocentric Ptolemaic model in favor of heliocentrism.

Greek cosmology reflects a blend of philosophical inquiry, empirical observation, and practical needs, shaping humanity’s early understanding of the universe and influencing scientific thought for centuries.