![]() ![]() Astronomers have known for a long time how much fainter things get with distance, and as with perspective it depends on the square of the distance. This is something we’re all familiar with from crossing roads at night – judging how far away a car is by how bright its headlights appear. The further away it is, the fainter it will be. The closer it is to me, the brighter it will appear. Now if extending arms wasn’t enough of a stretch for you, imagine that my thumbnail also glows. Twice the distance again and it would take sixteen thumbnails. If we imagine that my arm could grow to twice its length, then at twice the distance away it would take four of my thumbnails to block out the whole full Moon. ![]() The reason my single thumbnail can block out the whole Moon an arm’s length away from me on Earth, is all down to perspective. In fact, if I assume my thumbnail is roughly circular and about 1.5 centimetres across, I could fit 50,000,000,000,000,000 of my thumbnails on the surface of the Moon! We are all well aware that in reality the Moon is a lot bigger than your thumb. ![]() Try it next time you’re looking at the night sky and you’ll find you’ll be able to block the Moon out of the sky with your thumb. Imagine taking a small patch of sky, perhaps the size of your thumbnail, which held at arm’s length is about the size of the full Moon. This is called ‘ Olbers’ Paradox‘ if you want to read up on it. It should be shimmering brightly with the power of 200 sextillion distant suns. With this amount of stars in the universe our night sky shouldn’t be dark at all. The best estimate is 200 billion trillion stars or 200 sextillion. True in part, but what of the billions of stars astronomers tell us are in the universe? More stars than grains of sand on every beach and every desert. Therefore it gets dark at night because of the absence of sun light. The side facing the sun experiences day and the side facing away experiences night. The Earth rotates on its axis once every 24hrs or so. ![]() Surely this is the most simple question imaginable? Everyone knows why it gets dark at night don’t they?Īctually, it’s not quite as simple as you may think and it took modern science over 200 years and the writing of General Relativity to truly answer this question. Reading Time – 10 minutes, Difficulty Level 4/5 ![]()
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