Why we only recently discovered space is dark not bright

A blue Earth ascends over the barren surface of the moon, against the black void of space. This famous photograph, Earthrise, was taken on Christmas Eve of 1968, by Apollo 8 astronaut Bill Anders.

After almost six decades, we take this image for granted. But imagine a different Earthrise, in which space isn’t black but bright blue, like the clear day sky. As strange as it may strike you, this is how most Europeans imagined it for centuries.

We know our understanding of the universe has undergone other major transformations, with far-reaching effects. For example, the shifts from an Earth-centred to a sun-centred universe and from a finite to an infinite universe weren’t only scientific discoveries. They made people genuinely rethink their place in the cosmos. The shift from a bright to a dark universe is of comparable significance, but it has been almost lost to history.

In recent years, through my research in literary history and the history of science, I have tried to piece together when this shift happened. When, so to speak, did space turn dark? And I’ve found myself asking: what happened to us in the process?

Consider the testimony of Domingo Gonsales, the protagonist of the first English science-fiction novel, Francis Godwin’s 1638 Man in the Moone. Travelling to the moon aboard a swan-powered spacecraft, Gonsales reports seeing very few stars – and these few, “by reason it was always day, I saw at all times alike, not shining bright, as upon the earth we… see them in the night time, but of a whitish colour, like that of the moon in the day time with us”. Why does he see fewer stars than we do from Earth? And why are they pale, like the moon seen in the daytime sky? Because his space simply is the daytime sky. The sun has dimmed the light of the brightest stars and drowned out completely that of fainter ones.

From our perspective, Gonsales’s universe is upside down. In his version, it is in daytime that we see it as it really is, whereas at night it is obscured by Earth’s dark shadow. But if we ascended into space at midnight, we would eventually break out of the shadow, into the eternal day beyond.

Gonsales doesn’t mention the shadow, but we catch a glimpse of it in another early space travel story, John Milton’s Paradise Lost. Approaching Earth, Milton’s Satan sees “the circling canopy / Of night’s extended shade”. In imagining a premodern Earthrise, then, we should add this shadow into the picture – a dark cone extending from the gibbous planet into the blue heavens and disappearing below the lunar horizon.

Other authors explain why space isn’t just bright, but bright blue. The most common explanation is that the “firmament” – the variously imagined vault of the cosmos – was blue in colour. This is the view, notes Milton’s contemporary, the atomist philosopher Walter Charleton, held “not only by vulgar, but many transcendently learned heads”. In looking at the day sky, they thought they were simply looking at the end of the universe.

The path towards Earthrise

This universe also appears in visual art. Here, again, comparison with Apollo 8 is instructive. Some hours after capturing Earthrise, the crew delivered a radio broadcast to Earth from lunar orbit. Commander Frank Borman wished Earthlings a merry Christmas and read from the biblical account of creation. For the first time, humans attained a comparable, godlike perspective on their blue planet, sparkling in the black abyss. But when premodern artists illustrated these same biblical verses, they often drew the inverse: dark Earths, suspended in azure heavens. To complete the alternative Earthrise, imagine one of these darker Earths, rather than the familiar “blue marble”, ascending over the lunar surface.

And it wasn’t just poets and painters. Philosophers and scientists also imagined such universes. Aristotle describes “the shadow of the earth (which we call night)”. Two millennia later, so does Copernicus, writing that “while the rest of the universe is bright and full of daylight, night is clearly nothing but the Earth’s shadow, which extends in the shape of a cone and ends in a point”.

There was nothing irrational about such views. Early European thinkers simply had no compelling evidence to the contrary, especially regarding the nature of outer space and of Earth’s light-refracting