I think everyone at some point has looked up and asked "why is the sky blue?" , and if you go to the week of 3/27/11 in the archive you can find the answer! But have you ever looked up at the night sky and wondered why is the sky black? Well duh! It's because your not facing the sun, which lights up the sky during the day, right? Well, not exactly….
Think about this. The Universe is huge! Our galaxy is filled with hundreds of billions of bright stars, and the universe is filled with billions of bright galaxies. In every direction we look, there are bright sources of light, and this light has been traveling towards earth for over 14 billion years. So we would actually expect the night sky to be beaming with light from stars and galaxies! But obviously it's not, so what's going on? Any guesses?
Ok I'll tell you. The fact that the night sky is black is proof that our universe is expanding! What's happening is that as the universe expands, objects move away from us. The light emitted by these objects is Doppler shifted, which means it's shifted towards the "Red" part of the electromagnetic spectrum. The faster it moves, and the farther away the object is, the more red the object looks. Many of objects in the night sky are either so far away that they are too faint to be seen with the naked eye, or their light has been red-shifted into a part of the spectrum that our eyes are not sensitive to! And that is why we don't need "star-glasses" at night!
Image Credit: DeviantArt: ~andrei030
