CAELUM

Children draw the sky as a blue stripe along the top of the page. Between the stripe and the grass there is white paper, as if the sky were a ceiling and the air underneath were empty.

Adults know better, and still look up as if there were a lid. The blue seems to end somewhere. It has a colour, a height, a surface you could almost touch from an aeroplane window.

Anaximander, a pupil of Thales, thought about this more than two and a half thousand years ago. He was among the first to imagine that the earth hangs freely in space, held up by nothing. Behind everything we can see he placed the apeiron: the boundless, without edge or shape, from which all things come and into which they return.

The blue is light scattered in a thin layer of air. Above it the dark goes on further than anyone can picture. The ceiling was never there. It is simply the place where our eyes give up.

The sky is where our seeing stops. The world goes on.

There is a kind of freedom in knowing that. Most of what exists lies beyond what we can see, and this has always been true of more than the sky. Other people's lives, the future, the reasons behind things. We all live under a narrow blue stripe of what we understand.

On a clear day, standing in an open field, it is possible to feel this for a second. The blue opens, the ground feels very small, and that is fine.