0:00:10.170,0:00:11.172 Have you ever noticed 0:00:11.172,0:00:12.570 how the full moon looks bigger 0:00:12.570,0:00:14.042 when it's near the horizon 0:00:14.042,0:00:15.976 than when it's high over head? 0:00:15.976,0:00:17.530 If so, you're not alone. 0:00:17.530,0:00:19.730 People have wondered about this strange effect 0:00:19.730,0:00:21.105 since ancient times, 0:00:21.105,0:00:23.600 and surprisingly, we still don't have 0:00:23.600,0:00:24.315 a great explanation, 0:00:24.315,0:00:26.358 but that's not for lack of trying. 0:00:26.358,0:00:28.626 Some of the greatest minds in history - 0:00:28.626,0:00:29.395 Aristotle, 0:00:29.395,0:00:30.084 Ptolemy, 0:00:30.084,0:00:30.816 Da Vinci, 0:00:30.816,0:00:31.560 Decartes 0:00:31.560,0:00:33.286 - have all wrestled with this problem 0:00:33.286,0:00:36.645 and failed to generate an adequate explanation. 0:00:36.645,0:00:38.317 One of the first ideas suggested 0:00:38.317,0:00:40.426 was that the image of the moon in the sky 0:00:40.426,0:00:42.797 really is bigger near the horizon. 0:00:42.797,0:00:45.728 Perhaps the Earth's atmosphere acts like a giant lens, 0:00:45.728,0:00:48.343 magnifying the moon as it rises and sets. 0:00:48.343,0:00:50.680 But this explanation doesn't cut it. 0:00:50.680,0:00:52.816 If anything, the refraction of the atmosphere 0:00:52.816,0:00:55.100 would make the moon look slightly smaller. 0:00:55.100,0:00:56.561 Plus, if you actually measure 0:00:56.561,0:00:57.926 the size of the visible moon 0:00:57.926,0:00:59.940 at different positions, 0:00:59.940,0:01:00.594 it doesn't change at all. 0:01:00.594,0:01:02.438 But then, why does it still seem bigger 0:01:02.438,0:01:03.822 when it's rising? 0:01:03.822,0:01:06.141 This must be some kind of optical illusion. 0:01:06.141,0:01:08.207 The question is, which one? 0:01:08.207,0:01:11.113 One explanation is the Ebbinghaus Illusion, 0:01:11.113,0:01:13.327 where two identical objects look different 0:01:13.327,0:01:14.796 because of the relative size 0:01:14.796,0:01:17.100 of the objects they're surrounded by. 0:01:17.100,0:01:21.295 Here the two center circles are actually the same size. 0:01:21.295,0:01:23.323 Maybe the moon looks bigger near the horizon 0:01:23.323,0:01:25.206 because it's next to tiny trees, 0:01:25.206,0:01:25.843 houses, 0:01:25.843,0:01:27.680 and towers in the distance. 0:01:27.680,0:01:29.143 But when the moon is higher up, 0:01:29.143,0:01:31.648 it's surrounded by the vast darkness of the night sky 0:01:31.648,0:01:34.128 and looks tiny by comparison. 0:01:34.128,0:01:37.096 Another possibility is the famous Ponzo Illusion. 0:01:37.096,0:01:39.515 If you've ever tried to draw in perspective, 0:01:39.515,0:01:41.864 you know that the closer something is to the horizon, 0:01:41.864,0:01:43.818 the smaller you should draw it. 0:01:43.818,0:01:46.593 Our brain compensates automatically for this 0:01:46.593,0:01:48.706 by perceiving objects near the horizon 0:01:48.706,0:01:50.876 as larger than they actually appear. 0:01:50.876,0:01:52.787 The two yellow lines in this drawing 0:01:52.787,0:01:53.904 are the same size, 0:01:53.904,0:01:55.572 but the upper one seems bigger 0:01:55.572,0:01:57.412 because we interpret it as receding 0:01:57.412,0:01:59.042 farther into the horizon. 0:01:59.042,0:02:01.379 So, between Ponzo and Ebbinghaus, 0:02:01.379,0:02:02.911 it seems like we've solved the mystery 0:02:02.911,0:02:03.995 of the moon illusion, 0:02:03.995,0:02:06.375 but, unfortunately, there are a few details 0:02:06.375,0:02:08.354 that complicate things. 0:02:08.385,0:02:11.352 For one thing, if this was just the Ebbinghaus effect, 0:02:11.352,0:02:12.884 then we would expect the moon illusion 0:02:12.884,0:02:16.112 to disappear for pilots flying high above the clouds 0:02:16.112,0:02:18.066 since there wouldn't be any other smaller objects 0:02:18.066,0:02:19.599 near the horizon. 0:02:19.599,0:02:22.603 But in fact, pilots and sailors out on the ocean 0:02:22.603,0:02:24.825 still claim to see the moon illusion. 0:02:24.825,0:02:27.525 On the other hand, if it's just our brain's autocorrecting 0:02:27.525,0:02:29.663 the size of objects near the horizon, 0:02:29.663,0:02:31.196 then we'd expect the moon illusion 0:02:31.196,0:02:33.488 to be visible inside a planetarium, 0:02:33.488,0:02:34.449 where the whole sky, 0:02:34.449,0:02:35.619 including the horizon, 0:02:35.619,0:02:38.789 is displayed on a spherical dome overhead. 0:02:38.789,0:02:40.430 Studies have shown, though, 0:02:40.430,0:02:42.141 that this is not the case. 0:02:42.141,0:02:43.257 To make matters worse, 0:02:43.257,0:02:45.929 it seems the moon illusion disappears entirely 0:02:45.929,0:02:47.011 if you just bend over 0:02:47.011,0:02:49.071 and look at the moon between your legs. 0:02:49.071,0:02:51.379 Now, this is just getting ridiculous! 0:02:52.470,0:02:54.551 One of the most promising explanations today 0:02:54.551,0:02:57.384 is known as Convergence Micropsia. 0:02:57.384,0:02:59.568 Our brains judge the distance to objects 0:02:59.568,0:03:01.108 and their apparent size 0:03:01.108,0:03:03.064 by the focus of our eyes. 0:03:03.064,0:03:04.528 When looking at the horizon, 0:03:04.528,0:03:06.861 your eyes focus far off into the distance 0:03:06.861,0:03:09.200 so your brain knows you're looking far away. 0:03:09.200,0:03:11.178 The moon appears a certain size. 0:03:11.178,0:03:12.860 Your brain thinks it's far away, 0:03:12.860,0:03:13.705 which it is, 0:03:13.705,0:03:16.585 so you naturally conclude the moon must be big. 0:03:16.585,0:03:18.619 But when looking up at the night sky, 0:03:18.619,0:03:20.505 there's nothing for your eyes to focus on, 0:03:20.505,0:03:22.344 so they default to their rest focus, 0:03:22.344,0:03:25.261 which is a point just a few meters away. 0:03:25.261,0:03:27.185 Now your brain thinks the moon is much closer 0:03:27.185,0:03:28.343 than it really is, 0:03:28.343,0:03:29.430 so you naturally conclude 0:03:29.430,0:03:32.169 the moon's not as big as you thought it was. 0:03:32.169,0:03:33.811 Rather than explain why the moon 0:03:33.811,0:03:35.639 looks so big near the horizon, 0:03:35.639,0:03:37.432 Convergence Microspia explains 0:03:37.432,0:03:40.312 why the moon looks so small when overhead. 0:03:40.312,0:03:42.125 Still not satisfied? 0:03:42.125,0:03:44.851 Well, frankly, neither are many scientists, 0:03:44.851,0:03:47.841 so the debate over the moon illusion still rages on 0:03:47.841,0:03:50.346 and may continue as long as we still see it 0:03:50.346,0:03:51.440 in the night sky.