Why is the Sky Dark at Night?

Before I begin, I must first demonstrate the motivation behind this question, as many will doubtlessly come up with the common sense solution that the sun’s rays are not hitting our section of the planet. This, surprisingly enough, is not exactly the correct answer. In fact, as fellow science writer Julie San Fillipo pointed out in her article, the only reason the sky is blue during the day is because the photons (light particles) emitted from the sun are scattered in the earth’s atmosphere. If we did not have this atmosphere, the sky would always

[/media-credit] Although the sun does not appear in this image, we can tell from the way the earth is lit that the sun is shining on the moon as well, yet the sky remains black
be mostly dark. Here’s an example of this phenomenon: one may observe that on the surface of the moon, the sky (or whatever you want to call the area above the surface) is always dark even when the sun is shining on it.

One may note that in the previous sentence I said “mostly” dark and therein lays the actual motivation behind my original question: if the universe can be approximated as infinite (which it is), then shouldn’t it be true that if we look far enough in any direction we will eventually find a star just as bright as our sun? And if so shouldn’t the sky be entirely filled with light from those stars? The answer: sort of. Despite this ambiguous answer, our proposed query is not lost as we have found that even past the furthest stars and galaxies in the universe we still find light traveling toward us. But what is the origin of this mysterious light? It is the remnants of the earliest moment of the universe’s existence, the Big Bang. With this we have accomplished the first and most vital step in discovering knowledge about our physical universe: we know for sure what it is we do not know.

The reason the sky is not filled with star light is due to a phenomenon known as red-shifting. This process occurs due to the fact that the universe is in fact expanding and thus the distant stars from which the light originates are moving away from us. A simple every day example of this process can be demonstrated as such: when a police car drives by with a siren sounding, the siren sounds different when it is right next to you as opposed to when it is a good distance away and still moving yet further. So we have established that star light gets red-shifted, but why do we see still see some stars? It is because these are the stars closest to us and, consequently, the light undergoes red-shifting for a shorter amount of time. Correspondingly, the light from more distant stars gets more red-shifted until at some critical distance the light is shifted into the infrared frequency range and the human eye no longer detects it.

In summary, there is, in fact, light filling the sky in all directions at all times of the day, however we are simply unable to see most of it as our eyes have adjusted to detect only what we call the visible range (about 350nm-700nm). This means that if one looks with infrared goggles the sky will always be lit, as if one is wearing “night vision” goggles.



I must give credit for the inspiration and general explanation of this article to the podcast station “Physics Minute.” They have some extremely interesting discussions of physical concepts and explain in an easily graspable way so that anyway can enjoy in understanding the concepts. I strongly suggest checking them out, very cool stuff.

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