WEBVTT 99:59:59.999 --> 99:59:59.999 Have you ever noticed 99:59:59.999 --> 99:59:59.999 how the full moon looks bigger 99:59:59.999 --> 99:59:59.999 when it's near the horizon 99:59:59.999 --> 99:59:59.999 than when it's high over head? 99:59:59.999 --> 99:59:59.999 If so, you're not alone. 99:59:59.999 --> 99:59:59.999 People have wondered about this strange effect 99:59:59.999 --> 99:59:59.999 since ancient times, 99:59:59.999 --> 99:59:59.999 and surprisingly, we still don't have 99:59:59.999 --> 99:59:59.999 a great explanation, 99:59:59.999 --> 99:59:59.999 but that's not for lack of trying. 99:59:59.999 --> 99:59:59.999 Some of the greatest minds of history -- 99:59:59.999 --> 99:59:59.999 Aristotle, 99:59:59.999 --> 99:59:59.999 Ptolemy, 99:59:59.999 --> 99:59:59.999 Da Vinci, 99:59:59.999 --> 99:59:59.999 Decartes 99:59:59.999 --> 99:59:59.999 -- have all wrestled with this problem 99:59:59.999 --> 99:59:59.999 and failed to generate an adequate explanation. 99:59:59.999 --> 99:59:59.999 One of the first ideas suggested 99:59:59.999 --> 99:59:59.999 was that the image of the moon in the sky 99:59:59.999 --> 99:59:59.999 really is bigger near the horizon. 99:59:59.999 --> 99:59:59.999 Perhaps the Earth's atmosphere acts 99:59:59.999 --> 99:59:59.999 as a giant lens, 99:59:59.999 --> 99:59:59.999 magnifying the moon as it rises and sets. 99:59:59.999 --> 99:59:59.999 But this explanation doesn't cut it. 99:59:59.999 --> 99:59:59.999 If anything, the refraction of the atmosphere 99:59:59.999 --> 99:59:59.999 would make the moon look slightly smaller. 99:59:59.999 --> 99:59:59.999 Plus, if you actually measure 99:59:59.999 --> 99:59:59.999 the size of the visible moon 99:59:59.999 --> 99:59:59.999 at different positions, 99:59:59.999 --> 99:59:59.999 it doesn't change at all. 99:59:59.999 --> 99:59:59.999 But then, why does it still seem bigger 99:59:59.999 --> 99:59:59.999 when it's rising? 99:59:59.999 --> 99:59:59.999 This must be some kind of optical illusion. 99:59:59.999 --> 99:59:59.999 The question is, which one? 99:59:59.999 --> 99:59:59.999 One explanation is the Ebbinghaus Illusion, 99:59:59.999 --> 99:59:59.999 where two identical objects look different 99:59:59.999 --> 99:59:59.999 because of the relative size 99:59:59.999 --> 99:59:59.999 of the objects they're surrounded by. 99:59:59.999 --> 99:59:59.999 Here the two center circles are actually the same size. 99:59:59.999 --> 99:59:59.999 Maybe the moon looks bigger near the horizon 99:59:59.999 --> 99:59:59.999 because it's next to tiny trees, 99:59:59.999 --> 99:59:59.999 houses, 99:59:59.999 --> 99:59:59.999 and towers in the distance. 99:59:59.999 --> 99:59:59.999 But when the moon is higher up, 99:59:59.999 --> 99:59:59.999 it's surrounded by the vast darkness of the night sky 99:59:59.999 --> 99:59:59.999 and looks tiny by comparison. 99:59:59.999 --> 99:59:59.999 Another possibility is the famous Ponzo Illusion. 99:59:59.999 --> 99:59:59.999 If you've ever tried to draw in perspective, 99:59:59.999 --> 99:59:59.999 you know that the closer something is to the horizon, 99:59:59.999 --> 99:59:59.999 the smaller you should draw it. 99:59:59.999 --> 99:59:59.999 Our brain compensates automatically for this 99:59:59.999 --> 99:59:59.999 by perceiving objects near the horizon 99:59:59.999 --> 99:59:59.999 as larger than they actually appear. 99:59:59.999 --> 99:59:59.999 The two yellow lines in this drawing 99:59:59.999 --> 99:59:59.999 are the same size, 99:59:59.999 --> 99:59:59.999 but the upper one seems bigger 99:59:59.999 --> 99:59:59.999 because we interpret it as receding 99:59:59.999 --> 99:59:59.999 farther into the horizon. 99:59:59.999 --> 99:59:59.999 So, between Ponzo and Ebbinghaus, 99:59:59.999 --> 99:59:59.999 it seems like we've solved the mystery 99:59:59.999 --> 99:59:59.999 of the moon illusion, 99:59:59.999 --> 99:59:59.999 but, unfortunately, there are a few details 99:59:59.999 --> 99:59:59.999 that complicate things. 99:59:59.999 --> 99:59:59.999 For one thing, if this was just the Ebbinghaus effect, 99:59:59.999 --> 99:59:59.999 then we would expect the moon illusion 99:59:59.999 --> 99:59:59.999 to disappear for pilots flying high above the clouds 99:59:59.999 --> 99:59:59.999 since there wouldn't be any other smaller objects 99:59:59.999 --> 99:59:59.999 near the horizon. 99:59:59.999 --> 99:59:59.999 But in fact, pilots and sailors out on the ocean 99:59:59.999 --> 99:59:59.999 still claim to see the moon illusion. 99:59:59.999 --> 99:59:59.999 On the other hand, if it's just our brain's autocorrecting 99:59:59.999 --> 99:59:59.999 the size of objects near the horizon, 99:59:59.999 --> 99:59:59.999 then we'd expect the moon illusion 99:59:59.999 --> 99:59:59.999 to be visible inside a planetarium, 99:59:59.999 --> 99:59:59.999 where the whole sky, 99:59:59.999 --> 99:59:59.999 including the horizon, 99:59:59.999 --> 99:59:59.999 is displayed on a spherical dome overhead. 99:59:59.999 --> 99:59:59.999 Studies have shown, though, 99:59:59.999 --> 99:59:59.999 that this is not the case. 99:59:59.999 --> 99:59:59.999 To make matters worse, 99:59:59.999 --> 99:59:59.999 it seems the moon illusion disappears entirely 99:59:59.999 --> 99:59:59.999 if you just bend over 99:59:59.999 --> 99:59:59.999 and look at the moon between your legs. 99:59:59.999 --> 99:59:59.999 Now, this is just getting ridiculous! 99:59:59.999 --> 99:59:59.999 One of the most promising explanations today 99:59:59.999 --> 99:59:59.999 is known as Convergence Micropsia. 99:59:59.999 --> 99:59:59.999 Our brains judge the distance to objects 99:59:59.999 --> 99:59:59.999 and their apparent size 99:59:59.999 --> 99:59:59.999 by the focus of our eyes. 99:59:59.999 --> 99:59:59.999 When looking at the horizon, 99:59:59.999 --> 99:59:59.999 your eyes focus far-off into the distance 99:59:59.999 --> 99:59:59.999 so your brain knows you're looking far away. 99:59:59.999 --> 99:59:59.999 The moon appears a certain size. 99:59:59.999 --> 99:59:59.999 Your brain thinks it's far away, 99:59:59.999 --> 99:59:59.999 which it is, 99:59:59.999 --> 99:59:59.999 so you naturally conclude 99:59:59.999 --> 99:59:59.999 that the moon must be big. 99:59:59.999 --> 99:59:59.999 But when looking up at the night sky, 99:59:59.999 --> 99:59:59.999 there's nothing for your eyes to focus on, 99:59:59.999 --> 99:59:59.999 so they default to their rest focus, 99:59:59.999 --> 99:59:59.999 which is point just a few meters away. 99:59:59.999 --> 99:59:59.999 Now your brain thinks the moon is much closer 99:59:59.999 --> 99:59:59.999 than it really is, 99:59:59.999 --> 99:59:59.999 so you naturally conclude 99:59:59.999 --> 99:59:59.999 that the moon is not as big as you thought it was. 99:59:59.999 --> 99:59:59.999 Rather than explain why the moon 99:59:59.999 --> 99:59:59.999 looks so big near the horizon, 99:59:59.999 --> 99:59:59.999 Convergence Microspia explains 99:59:59.999 --> 99:59:59.999 why the moon looks so small when overhead. 99:59:59.999 --> 99:59:59.999 Still not satisfied? 99:59:59.999 --> 99:59:59.999 Well, frankly, neither are many scientists 99:59:59.999 --> 99:59:59.999 so the debate over the moon illusion still rages on 99:59:59.999 --> 99:59:59.999 and may continue as long as we still see it 99:59:59.999 --> 99:59:59.999 in the night sky.