How Far Can We See?

How Far Can We See?

The father of modern science, Isaac Newton, once said “If I have seen further, it is by standing on the shoulders of giants”. He was making the point that human knowledge grows incrementally, or that modern advancements are built upon earlier ideas. Taken literally, despite referring to huge mythical creatures that smell Englishmen, his quote makes scientific sense. The higher you are, the further you see.

Someone standing at sea level sees the horizon bend away at about five kilometres. Math fundis will tell you that the horizon for someone standing at the peak of a mountain would be much further away. In either case, they would have no trouble seeing the horizon. This brings to mind the question: How Far Can We See?

Scientists have found the absolute minimum amount of light needed to hit our eyes for us to register seeing something corresponds to a candle flickering at a distance of fifty kilometres. It’s not a stretch, however, to assume that the candle flickering at fifty kilometres wouldn’t really look very clear. How far before something on our scale becomes indistinguishable? It turns out that this distance is much, much shorter (even closer than the horizon). Three kilometres is the limit, which makes sense when you think about the distance away that an approaching car has to be before that single headlight becomes two distinguishable spots.

We do, however, see objects much, much farther away on an almost nightly basis. The furthest star that is visible to the eye is Cassiopeia, which is 16308 light years away. To put that into perspective, that means that light leaving the surface of that star will travel at a speed of 300000 kilometres per second for 16308 years before reaching our eyeballs. Since the human race first laid eyes on Cassiopeia, we’ve wondered about the existence of others like us. Or places like Earth.

The Kepler space telescope is a space observatory whose sole mission is to identify whether there are any Earth-size planets orbiting their stars at just the right distance to support liquid water pooling on the surface. It was launched in 2009, and has recently released a catalogues of 219 new planet candidates, 10 of which are Earth-sized with just the right orbit. These findings could have significant implications in our search for life elsewhere.

The vision of the first cave-dwellers who peered up at the night sky questioning, that of Isaac Newton who looked towards the horizon on those metaphoric shoulders, and of those scientists working on Kepler, amongst many others all enable us to see as far as we have. And to wonder whether humanity’s vision is, in fact, boundless.

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