The heavens as air made solid

Empedocles, a fifth-century BCE Greek poet and natural philosopher from Acragas in Sicily, is credited in the ancient tradition with a surprising explanation of the heavens. A report preserved by Eusebius of Caesarea among the opinions of earlier thinkers describes the sky as a solid body. The brief form of the report matters for its interpretation. It gives us a view attributed to Empedocles. The words at this point belong to the transmission of his thought rather than to a surviving line of his poetry.

A vault above the observer

Empedocles: the heaven is solid, formed from air compressed by fire into a crystallized form, and encompassing the whole elements of fire and air in each of the hemispheres.

– Empedocles, Testimony in Eusebius, 1903, p. 908.^[1]

The comparison is to air made solid in a crystalline fashion through the action of fire. Two experiences meet that might appear difficult to reconcile at a first glance upwards. To look at the sky is to perceive a distance in which nothing seems to stop the eye. To imagine a vault overhead, on the other hand, is to picture a boundary capable of enduring. A clear crystal can show both things on a small scale. It is firm while leaving the view through it open.

Why crystal supplies the comparison

The striking move is from the nearby object to something exceedingly remote. Nobody can touch the supposed surface of heaven or turn it around against the light. Crystal nevertheless supplies an experience from which the idea can be developed. The sentence preserves more than an ancient proposal about the structure of the cosmos. It shows how the properties of a small body could provide language in which to think about the whole of the visible sky.