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