Celestial spheres

Lore & Background

In Greek antiquity the ideas of celestial spheres and rings first appeared in the cosmology of Anaximander in the early 6th century BC. In his cosmology both the Sun and Moon are circular open vents in tubular rings of fire enclosed in tubes of condensed air; these rings constitute the rims of rotating chariot-like wheels pivoting on the Earth at their centre. The fixed stars are also open vents in such wheel rims, but there are so many such wheels for the stars that their contiguous rims all together form a continuous spherical shell encompassing the Earth.

All these wheel rims had originally been formed out of an original sphere of fire wholly encompassing the Earth, which had disintegrated into many individual rings. Hence, in Anaximander's cosmogony, in the beginning was the sphere, out of which celestial rings were formed, from some of which the stellar sphere was in turn composed. As viewed from the Earth, the ring of the Sun was highest, that of the Moon was lower, and the sphere of the stars was lowest.

Following Anaximander, his pupil Anaximenes held that the stars, Sun, Moon, and planets are all made of fire. But whilst the stars are fastened on a revolving crystal sphere like nails or studs, the Sun, Moon, and planets, and also the Earth, all just ride on air like leaves because of their breadth. And whilst the fixed stars are carried around in a complete circle by the stellar sphere, the Sun, Moon and planets do not revolve under the Earth between setting and rising again like the stars do, but rather on setting they go laterally around the Earth like a cap turning halfway around the head until they rise again. And unlike Anaximander, he relegated the fixed stars to the region most distant from the Earth.

The most enduring feature of Anaximenes' cosmos was its conception of the stars being fixed on a crystal sphere as in a rigid frame, which became a fundamental principle of cosmology down to Copernicus and Kepler. And much later in the fourth century BC Plato's Timaeus proposed that the body of the cosmos was made in the most perfect and uniform shape, that of a sphere containing the fixed stars. But it posited that the planets were spherical bodies set in rotating bands or rings rather than wheel rims as in Anaximander's cosmology.

Roots in Early Greek Cosmology

In the earliest Greek attempts to explain the heavens, the concept of celestial spheres emerged through a sequence of significant proposals. Anaximander, writing in the early sixth century BC, pictured the Sun and Moon as circular openings in rings of fire within tubes of compressed air, while the fixed stars were apertures in the rims of countless rotating wheels. Because so many stellar wheels existed, their rims merged into one unbroken spherical shell around the Earth. He traced all these rings back to a primordial sphere of fire that had shattered into individual circles.

His student Anaximenes refined the picture by fixing the stars onto a revolving crystal sphere, like nails set into a rigid frame, while the Sun, Moon, and planets drifted on air like broad leaves. This image of stars locked into a crystalline shell proved remarkably durable, persisting as a foundational assumption in cosmology through Copernicus and Kepler. Pythagoras, Xenophanes, and Parmenides all affirmed the spherical shape of the universe, and Plato's Timaeus codified the notion that the cosmos takes the form of a perfect sphere containing the fixed stars, with planets riding in rotating bands rather than the older wheel-rim construction.

The Physical Architecture of Nested Orbs

Unlike the modern view of planets tracing paths through mostly empty space, ancient and medieval thinkers understood the celestial orbs as thick, tangible spheres of rarefied matter. Each orb was conceived as a transparent shell of quintessence, the aetherial fifth element, nested snugly within the next, with every sphere in complete contact with both its neighbor above and below. The fixed stars, believed to hold unchanging positions relative to one another, were argued to sit on the surface of a single starry sphere, like gems set into a great orb.

When scholars applied Ptolemy's epicycles, they assumed each planetary sphere was precisely thick enough to accommodate those complex motions. By combining this nested architecture with observational data, they derived what became the generally accepted distances of the era: roughly four million miles to the Sun and about seventy-three million miles to the outer edge of the universe. Albert Van Helden has noted that from around 1250 until the seventeenth century, virtually every educated European was familiar with this Ptolemaic scheme and the cosmic dimensions it implied.

From Bands to Concentric Spheres

Plato's student Eudoxus transformed the planetary model by replacing the earlier band-and-ring imagery with a system of concentric spheres assigned to every planet. He allocated three spheres each to the Moon and the Sun, and four to each of the remaining five planets, yielding a total of twenty-six spheres. Callippus later revised the scheme, increasing the count to five spheres for the Sun, Moon, Mercury, Venus, and Mars while keeping four for Jupiter and Saturn, bringing the total to thirty-three. In both systems, each planet was attached to the innermost sphere of its own set.

Although these models captured the major qualitative features of planetary motion, they could not deliver precise quantitative predictions. Historians of Greek science have long debated whether Eudoxus and Callippus intended their spheres as mere geometrical constructs or as physically real entities, and the surviving evidence has not resolved the question. Aristotle, in his Metaphysics, built a fully physical cosmology on Eudoxus's mathematical foundation, placing the spherical Earth at the center of the universe with the planets moved by the nested spheres.

The Dissolution of the Spheres

Mainstream belief in the celestial sphere framework did not survive the Scientific Revolution. In the early 1600s, Kepler still discussed celestial spheres in his writings, yet he no longer held that planets were physically carried by them. Instead, he proposed that the planets travel along elliptical paths governed by what are now called Kepler's laws of planetary motion. By the late 1600s, the Greek and medieval theories concerning the motion of both terrestrial and celestial objects were effectively supplanted by Newton's law of universal gravitation and the broader framework of Newtonian mechanics.

Newton's theory explained how Kepler's empirical laws arise naturally from the gravitational attraction between bodies, rendering the physical spheres unnecessary. Even Copernicus, while shifting the Sun to the center, had retained a version of the nested sphere model, ordering the planetary spheres as Mercury, Venus, Earth-Moon, Mars, Jupiter, and Saturn. Today, the orbits of the planets are understood simply as trajectories through largely empty space, and the universe is known to be inconceivably vast and still expanding.

Reader's Guide

In modern thought, the orbits of the planets are viewed as the paths of those planets through mostly empty space. Ancient and medieval thinkers, however, considered the celestial orbs to be thick spheres of rarefied matter nested one within the other, each one in complete contact with the sphere above it and the sphere below. When scholars applied Ptolemy's epicycles, they presumed that each planetary sphere was exactly thick enough to accommodate them.

By combining this nested sphere model with astronomical observations, scholars calculated what became generally accepted values at the time for the distances to the Sun: about 4 e6mi, to the other planets, and to the edge of the universe: about 73 e6mi. The nested sphere model's distances to the Sun and planets differ significantly from modern measurements of the distances, and the size of the universe is now known to be inconceivably large and continuously expanding. Albert Van Helden has suggested that from about 1250 until the 17th century, virtually all educated Europeans were familiar with the Ptolemaic model of 'nesting spheres and the cosmic dimensions derived from it'. Even after Copernicus published his heliostatic model of the universe, new versions of the celestial sphere model were introduced, with the planetary spheres following this sequence from the Sun at the centre: Mercury, Venus, Earth-Moon, Mars, Jupiter and Saturn.

Mainstream belief in the theory of celestial spheres did not survive the Scientific Revolution. In the early 1600s, Kepler continued to discuss celestial spheres, although he did not consider that the planets were carried by the spheres but held that they moved in elliptical paths described by Kepler's laws of planetary motion. In the late 1600s, Greek and medieval theories concerning the motion of terrestrial and celestial objects were replaced by Newton's law of universal gravitation and Newtonian mechanics, which explain how Kepler's laws arise from the gravitational attraction between bodies.