1 00:00:10,170 --> 00:00:11,172 Have you ever noticed 2 00:00:11,172 --> 00:00:12,570 how the full moon looks bigger 3 00:00:12,570 --> 00:00:14,042 when it's near the horizon 4 00:00:14,042 --> 00:00:15,976 than when it's high over head? 5 00:00:15,976 --> 00:00:17,530 If so, you're not alone. 6 00:00:17,530 --> 00:00:19,730 People have wondered about this strange effect 7 00:00:19,730 --> 00:00:21,105 since ancient times, 8 00:00:21,105 --> 00:00:23,600 and surprisingly, we still don't have 9 00:00:23,600 --> 00:00:24,315 a great explanation, 10 00:00:24,315 --> 00:00:26,358 but that's not for lack of trying. 11 00:00:26,358 --> 00:00:28,626 Some of the greatest minds in history - 12 00:00:28,626 --> 00:00:29,395 Aristotle, 13 00:00:29,395 --> 00:00:30,084 Ptolemy, 14 00:00:30,084 --> 00:00:30,816 Da Vinci, 15 00:00:30,816 --> 00:00:31,560 Decartes 16 00:00:31,560 --> 00:00:33,286 - have all wrestled with this problem 17 00:00:33,286 --> 00:00:36,645 and failed to generate an adequate explanation. 18 00:00:36,645 --> 00:00:38,317 One of the first ideas suggested 19 00:00:38,317 --> 00:00:40,426 was that the image of the moon in the sky 20 00:00:40,426 --> 00:00:42,797 really is bigger near the horizon. 21 00:00:42,797 --> 00:00:45,728 Perhaps the Earth's atmosphere acts like a giant lens, 22 00:00:45,728 --> 00:00:48,343 magnifying the moon as it rises and sets. 23 00:00:48,343 --> 00:00:50,680 But this explanation doesn't cut it. 24 00:00:50,680 --> 00:00:52,816 If anything, the refraction of the atmosphere 25 00:00:52,816 --> 00:00:55,100 would make the moon look slightly smaller. 26 00:00:55,100 --> 00:00:56,561 Plus, if you actually measure 27 00:00:56,561 --> 00:00:57,926 the size of the visible moon 28 00:00:57,926 --> 00:00:59,940 at different positions, 29 00:00:59,940 --> 00:01:00,594 it doesn't change at all. 30 00:01:00,594 --> 00:01:02,438 But then, why does it still seem bigger 31 00:01:02,438 --> 00:01:03,822 when it's rising? 32 00:01:03,822 --> 00:01:06,141 This must be some kind of optical illusion. 33 00:01:06,141 --> 00:01:08,207 The question is, which one? 34 00:01:08,207 --> 00:01:11,113 One explanation is the Ebbinghaus Illusion, 35 00:01:11,113 --> 00:01:13,327 where two identical objects look different 36 00:01:13,327 --> 00:01:14,796 because of the relative size 37 00:01:14,796 --> 00:01:17,100 of the objects they're surrounded by. 38 00:01:17,100 --> 00:01:21,295 Here the two center circles are actually the same size. 39 00:01:21,295 --> 00:01:23,323 Maybe the moon looks bigger near the horizon 40 00:01:23,323 --> 00:01:25,206 because it's next to tiny trees, 41 00:01:25,206 --> 00:01:25,843 houses, 42 00:01:25,843 --> 00:01:27,680 and towers in the distance. 43 00:01:27,680 --> 00:01:29,143 But when the moon is higher up, 44 00:01:29,143 --> 00:01:31,648 it's surrounded by the vast darkness of the night sky 45 00:01:31,648 --> 00:01:34,128 and looks tiny by comparison. 46 00:01:34,128 --> 00:01:37,096 Another possibility is the famous Ponzo Illusion. 47 00:01:37,096 --> 00:01:39,515 If you've ever tried to draw in perspective, 48 00:01:39,515 --> 00:01:41,864 you know that the closer something is to the horizon, 49 00:01:41,864 --> 00:01:43,818 the smaller you should draw it. 50 00:01:43,818 --> 00:01:46,593 Our brain compensates automatically for this 51 00:01:46,593 --> 00:01:48,706 by perceiving objects near the horizon 52 00:01:48,706 --> 00:01:50,876 as larger than they actually appear. 53 00:01:50,876 --> 00:01:52,787 The two yellow lines in this drawing 54 00:01:52,787 --> 00:01:53,904 are the same size, 55 00:01:53,904 --> 00:01:55,572 but the upper one seems bigger 56 00:01:55,572 --> 00:01:57,412 because we interpret it as receding 57 00:01:57,412 --> 00:01:59,042 farther into the horizon. 58 00:01:59,042 --> 00:02:01,379 So, between Ponzo and Ebbinghaus, 59 00:02:01,379 --> 00:02:02,911 it seems like we've solved the mystery 60 00:02:02,911 --> 00:02:03,995 of the moon illusion, 61 00:02:03,995 --> 00:02:06,375 but, unfortunately, there are a few details 62 00:02:06,375 --> 00:02:08,354 that complicate things. 63 00:02:08,385 --> 00:02:11,352 For one thing, if this was just the Ebbinghaus effect, 64 00:02:11,352 --> 00:02:12,884 then we would expect the moon illusion 65 00:02:12,884 --> 00:02:16,112 to disappear for pilots flying high above the clouds 66 00:02:16,112 --> 00:02:18,066 since there wouldn't be any other smaller objects 67 00:02:18,066 --> 00:02:19,599 near the horizon. 68 00:02:19,599 --> 00:02:22,603 But in fact, pilots and sailors out on the ocean 69 00:02:22,603 --> 00:02:24,825 still claim to see the moon illusion. 70 00:02:24,825 --> 00:02:27,525 On the other hand, if it's just our brain's autocorrecting 71 00:02:27,525 --> 00:02:29,663 the size of objects near the horizon, 72 00:02:29,663 --> 00:02:31,196 then we'd expect the moon illusion 73 00:02:31,196 --> 00:02:33,488 to be visible inside a planetarium, 74 00:02:33,488 --> 00:02:34,449 where the whole sky, 75 00:02:34,449 --> 00:02:35,619 including the horizon, 76 00:02:35,619 --> 00:02:38,789 is displayed on a spherical dome overhead. 77 00:02:38,789 --> 00:02:40,430 Studies have shown, though, 78 00:02:40,430 --> 00:02:42,141 that this is not the case. 79 00:02:42,141 --> 00:02:43,257 To make matters worse, 80 00:02:43,257 --> 00:02:45,929 it seems the moon illusion disappears entirely 81 00:02:45,929 --> 00:02:47,011 if you just bend over 82 00:02:47,011 --> 00:02:49,071 and look at the moon between your legs. 83 00:02:49,071 --> 00:02:51,379 Now, this is just getting ridiculous! 84 00:02:52,470 --> 00:02:54,551 One of the most promising explanations today 85 00:02:54,551 --> 00:02:57,384 is known as Convergence Micropsia. 86 00:02:57,384 --> 00:02:59,568 Our brains judge the distance to objects 87 00:02:59,568 --> 00:03:01,108 and their apparent size 88 00:03:01,108 --> 00:03:03,064 by the focus of our eyes. 89 00:03:03,064 --> 00:03:04,528 When looking at the horizon, 90 00:03:04,528 --> 00:03:06,861 your eyes focus far off into the distance 91 00:03:06,861 --> 00:03:09,200 so your brain knows you're looking far away. 92 00:03:09,200 --> 00:03:11,178 The moon appears a certain size. 93 00:03:11,178 --> 00:03:12,860 Your brain thinks it's far away, 94 00:03:12,860 --> 00:03:13,705 which it is, 95 00:03:13,705 --> 00:03:16,585 so you naturally conclude the moon must be big. 96 00:03:16,585 --> 00:03:18,619 But when looking up at the night sky, 97 00:03:18,619 --> 00:03:20,505 there's nothing for your eyes to focus on, 98 00:03:20,505 --> 00:03:22,344 so they default to their rest focus, 99 00:03:22,344 --> 00:03:25,261 which is a point just a few meters away. 100 00:03:25,261 --> 00:03:27,185 Now your brain thinks the moon is much closer 101 00:03:27,185 --> 00:03:28,343 than it really is, 102 00:03:28,343 --> 00:03:29,430 so you naturally conclude 103 00:03:29,430 --> 00:03:32,169 the moon's not as big as you thought it was. 104 00:03:32,169 --> 00:03:33,811 Rather than explain why the moon 105 00:03:33,811 --> 00:03:35,639 looks so big near the horizon, 106 00:03:35,639 --> 00:03:37,432 Convergence Microspia explains 107 00:03:37,432 --> 00:03:40,312 why the moon looks so small when overhead. 108 00:03:40,312 --> 00:03:42,125 Still not satisfied? 109 00:03:42,125 --> 00:03:44,851 Well, frankly, neither are many scientists, 110 00:03:44,851 --> 00:03:47,841 so the debate over the moon illusion still rages on 111 00:03:47,841 --> 00:03:50,346 and may continue as long as we still see it 112 00:03:50,346 --> 00:03:51,440 in the night sky.