HomeThe shape of the sky
Or, confusing perceptions
This is an astronomy post, but it’s also not really an astronomy post; it’s more a post about me and the way I think.
When I was small, I was terrified by the size of the universe. I can remember, about seven or eight or so, really struggling with the concept that the universe might be infinite and might not be, and I can still remember the mental picture I tried to come up with of a universe where the stars just, at some point, stopped; and beyond there was just blackness.
It probably doesn’t work like that, but still in my head somewhere is the concept that it might do. Moreover, I have trouble with another, broader concept, which is that—assuming you can travel between both hemispheres—we can “see” all of the observable universe.
“See” is in quotes there because, well, you can’t see everything for practical reasons. Most things are too faint, firstly. At any given time half the sky is too close to the Sun, too: you can’t see Mercury right now for example. But in theory, barring things being too faint, barring you having to wait for the Earth to move a bit or having to travel from one side of the Equator to the other, the whole universe is up above some point on the planet’s surface at any given time.
This probably seems tediously self-evident if you think of the planet as a ball spinning through space. For some reason, though, the whole concept still catches me by surprise occasionally. I still think that things must be able to, I don’t know, hide around a corner or something like that. I think I must have picked up the idea from a book I had as a child about Halley’s Comet, which included diagrams of where you would be able to see the comet in the sky in 1986, and talked about how comets appeared and disappeared in the sky. That’s the general tone when talking about comets and asteroids and so on: they appear in the sky and they disappear again. So it took me a long time to realise that all of the comets and all of the asteroids are up there, in front of us, all along; we just can’t see them right now. Halley’s comet doesn’t just pop out from behind a tree every seventy-five years: it’s up there in the sky the whole time, just not visible.
Halley’s comet is maybe a bad example for this. Because it’s so famous, and because of the light-gathering power of modern telescopes, we can now track it through its entire orbit. According to Stellarium it’s currently at coordinates 8h26m/+1°46’, but as it’s only a few years from perigee it’s an incredibly-faint magnitude 25.5 so will appear as a just a fuzzy handful of pixels on any photo. Nevertheless, if you go outside tonight and look up at the constellation Hydra close to its tripoint with Monoceros and Canis Minor, up there it is. As are all the others. Everything in the sky that you’re likely to see in your lifetime is already up there in the sky, just invisible and unrecognised. And despite the fact that I know this, that I know on a rough, superficial level how the mechanics of cosmology work, it still feels a little strange to me. It still feels in my mind as if there should be some patch of the sky that we can’t see, that is hiding around some sort of galactic corner.
Whenever I see diagrams of the whole sky that say “this is the whole universe”—cosmic background radiation maps, for example—I’m slightly disturbed that in some sense the whole universe fits into one small image. “Surely it must be bigger?” my mind ends up thinking. The edges trick me: that’s not the edge of the universe, it’s just an artifact of plotting a spherical sky onto a flat piece of paper, just like any sort of atlas. The mental disconnect, though, leaves me feeling deeply uncomfortable.
Really, what I feel I should inspire me here is to take away that the night sky is far more mysterious and secretive than we know. It’s not just laid out flat in front of us as it appears to be: it’s full of unknown things and unanswered questions, even though all of them are genuinely sitting right there up above us somewhere. I don’t know if I’ll ever manage to change the way my head thinks about the night sky, but if I can, there is a whole universe of wonder concealed but fully within sight.