9:59:59.000,9:59:59.000 Have you ever noticed 9:59:59.000,9:59:59.000 how the full moon looks bigger 9:59:59.000,9:59:59.000 when it's near the horizon 9:59:59.000,9:59:59.000 than when it's high over head? 9:59:59.000,9:59:59.000 If so, you're not alone. 9:59:59.000,9:59:59.000 People have wondered about this strange effect 9:59:59.000,9:59:59.000 since ancient times, 9:59:59.000,9:59:59.000 and surprisingly, we still don't have 9:59:59.000,9:59:59.000 a great explanation, 9:59:59.000,9:59:59.000 but that's not for lack of trying. 9:59:59.000,9:59:59.000 Some of the greatest minds of history -- 9:59:59.000,9:59:59.000 Aristotle, 9:59:59.000,9:59:59.000 Ptolemy, 9:59:59.000,9:59:59.000 Da Vinci, 9:59:59.000,9:59:59.000 Decartes 9:59:59.000,9:59:59.000 -- have all wrestled with this problem 9:59:59.000,9:59:59.000 and failed to generate an adequate explanation. 9:59:59.000,9:59:59.000 One of the first ideas suggested 9:59:59.000,9:59:59.000 was that the image of the moon in the sky 9:59:59.000,9:59:59.000 really is bigger near the horizon. 9:59:59.000,9:59:59.000 Perhaps the Earth's atmosphere acts 9:59:59.000,9:59:59.000 as a giant lens, 9:59:59.000,9:59:59.000 magnifying the moon as it rises and sets. 9:59:59.000,9:59:59.000 But this explanation doesn't cut it. 9:59:59.000,9:59:59.000 If anything, the refraction of the atmosphere 9:59:59.000,9:59:59.000 would make the moon look slightly smaller. 9:59:59.000,9:59:59.000 Plus, if you actually measure 9:59:59.000,9:59:59.000 the size of the visible moon 9:59:59.000,9:59:59.000 at different positions, 9:59:59.000,9:59:59.000 it doesn't change at all. 9:59:59.000,9:59:59.000 But then, why does it still seem bigger 9:59:59.000,9:59:59.000 when it's rising? 9:59:59.000,9:59:59.000 This must be some kind of optical illusion. 9:59:59.000,9:59:59.000 The question is, which one? 9:59:59.000,9:59:59.000 One explanation is the Ebbinghaus Illusion, 9:59:59.000,9:59:59.000 where two identical objects look different 9:59:59.000,9:59:59.000 because of the relative size 9:59:59.000,9:59:59.000 of the objects they're surrounded by. 9:59:59.000,9:59:59.000 Here the two center circles are actually the same size. 9:59:59.000,9:59:59.000 Maybe the moon looks bigger near the horizon 9:59:59.000,9:59:59.000 because it's next to tiny trees, 9:59:59.000,9:59:59.000 houses, 9:59:59.000,9:59:59.000 and towers in the distance. 9:59:59.000,9:59:59.000 But when the moon is higher up, 9:59:59.000,9:59:59.000 it's surrounded by the vast darkness of the night sky 9:59:59.000,9:59:59.000 and looks tiny by comparison. 9:59:59.000,9:59:59.000 Another possibility is the famous Ponzo Illusion. 9:59:59.000,9:59:59.000 If you've ever tried to draw in perspective, 9:59:59.000,9:59:59.000 you know that the closer something is to the horizon, 9:59:59.000,9:59:59.000 the smaller you should draw it. 9:59:59.000,9:59:59.000 Our brain compensates automatically for this 9:59:59.000,9:59:59.000 by perceiving objects near the horizon 9:59:59.000,9:59:59.000 as larger than they actually appear. 9:59:59.000,9:59:59.000 The two yellow lines in this drawing 9:59:59.000,9:59:59.000 are the same size, 9:59:59.000,9:59:59.000 but the upper one seems bigger 9:59:59.000,9:59:59.000 because we interpret it as receding 9:59:59.000,9:59:59.000 farther into the horizon. 9:59:59.000,9:59:59.000 So, between Ponzo and Ebbinghaus, 9:59:59.000,9:59:59.000 it seems like we've solved the mystery 9:59:59.000,9:59:59.000 of the moon illusion, 9:59:59.000,9:59:59.000 but, unfortunately, there are a few details 9:59:59.000,9:59:59.000 that complicate things. 9:59:59.000,9:59:59.000 For one thing, if this was just the Ebbinghaus effect, 9:59:59.000,9:59:59.000 then we would expect the moon illusion 9:59:59.000,9:59:59.000 to disappear for pilots flying high above the clouds 9:59:59.000,9:59:59.000 since there wouldn't be any other smaller objects 9:59:59.000,9:59:59.000 near the horizon. 9:59:59.000,9:59:59.000 But in fact, pilots and sailors out on the ocean 9:59:59.000,9:59:59.000 still claim to see the moon illusion. 9:59:59.000,9:59:59.000 On the other hand, if it's just our brain's autocorrecting 9:59:59.000,9:59:59.000 the size of objects near the horizon, 9:59:59.000,9:59:59.000 then we'd expect the moon illusion 9:59:59.000,9:59:59.000 to be visible inside a planetarium, 9:59:59.000,9:59:59.000 where the whole sky, 9:59:59.000,9:59:59.000 including the horizon, 9:59:59.000,9:59:59.000 is displayed on a spherical dome overhead. 9:59:59.000,9:59:59.000 Studies have shown, though, 9:59:59.000,9:59:59.000 that this is not the case. 9:59:59.000,9:59:59.000 To make matters worse, 9:59:59.000,9:59:59.000 it seems the moon illusion disappears entirely 9:59:59.000,9:59:59.000 if you just bend over 9:59:59.000,9:59:59.000 and look at the moon between your legs. 9:59:59.000,9:59:59.000 Now, this is just getting ridiculous! 9:59:59.000,9:59:59.000 One of the most promising explanations today 9:59:59.000,9:59:59.000 is known as Convergence Micropsia. 9:59:59.000,9:59:59.000 Our brains judge the distance to objects 9:59:59.000,9:59:59.000 and their apparent size 9:59:59.000,9:59:59.000 by the focus of our eyes. 9:59:59.000,9:59:59.000 When looking at the horizon, 9:59:59.000,9:59:59.000 your eyes focus far-off into the distance 9:59:59.000,9:59:59.000 so your brain knows you're looking far away. 9:59:59.000,9:59:59.000 The moon appears a certain size. 9:59:59.000,9:59:59.000 Your brain thinks it's far away, 9:59:59.000,9:59:59.000 which it is, 9:59:59.000,9:59:59.000 so you naturally conclude 9:59:59.000,9:59:59.000 that the moon must be big. 9:59:59.000,9:59:59.000 But when looking up at the night sky, 9:59:59.000,9:59:59.000 there's nothing for your eyes to focus on, 9:59:59.000,9:59:59.000 so they default to their rest focus, 9:59:59.000,9:59:59.000 which is point just a few meters away. 9:59:59.000,9:59:59.000 Now your brain thinks the moon is much closer 9:59:59.000,9:59:59.000 than it really is, 9:59:59.000,9:59:59.000 so you naturally conclude 9:59:59.000,9:59:59.000 that the moon is not as big as you thought it was. 9:59:59.000,9:59:59.000 Rather than explain why the moon 9:59:59.000,9:59:59.000 looks so big near the horizon, 9:59:59.000,9:59:59.000 Convergence Microspia explains 9:59:59.000,9:59:59.000 why the moon looks so small when overhead. 9:59:59.000,9:59:59.000 Still not satisfied? 9:59:59.000,9:59:59.000 Well, frankly, neither are many scientists 9:59:59.000,9:59:59.000 so the debate over the moon illusion still rages on 9:59:59.000,9:59:59.000 and may continue as long as we still see it 9:59:59.000,9:59:59.000 in the night sky.