WEBVTT 00:00:00.000 --> 00:00:03.180 So I want you to picture yourself on a just a 00:00:03.180 --> 00:00:05.960 really warm, lazy afternoon. You're sitting right 00:00:05.960 --> 00:00:08.960 under this large leafy tree. Oh, nice. Setting 00:00:08.960 --> 00:00:11.279 the scene. Right. And if you look down at the 00:00:11.279 --> 00:00:13.919 footpath around you, you'll see what you always 00:00:13.919 --> 00:00:17.260 see under a tree on a sunny day. Just thousands 00:00:17.260 --> 00:00:21.440 of those ordinary scattered round patches of 00:00:21.440 --> 00:00:23.839 dappled light filtering down through the canopy. 00:00:23.899 --> 00:00:27.000 Yeah, totally mundane. everyday physics exactly 00:00:27.000 --> 00:00:29.320 it's a completely mundane site but now picture 00:00:29.320 --> 00:00:31.899 that exact same afternoon that same tree but 00:00:31.899 --> 00:00:35.079 uh during a partial solar eclipse oh things get 00:00:35.079 --> 00:00:37.679 weird they get so weird the moon has just taken 00:00:37.679 --> 00:00:39.759 a little bite out of the sun's disc up in the 00:00:39.759 --> 00:00:42.060 sky right and if you look down at the footpath 00:00:42.060 --> 00:00:45.420 now suddenly All those patches of light are not 00:00:45.420 --> 00:00:48.219 round anymore. Not at all. Every single gap between 00:00:48.219 --> 00:00:50.579 the leaves above you has completely transformed. 00:00:50.820 --> 00:00:53.020 You are looking at a footpath scattered with 00:00:53.020 --> 00:00:55.740 thousands of tiny repeated crescents. It's honestly 00:00:55.740 --> 00:00:58.500 one of the most beautiful accidental science 00:00:58.500 --> 00:01:00.859 demonstrations in all of nature. Right, because 00:01:00.859 --> 00:01:03.579 it is a perfect miniature copy of whatever shape 00:01:03.579 --> 00:01:06.579 the sun currently is, just repeated over and 00:01:06.579 --> 00:01:08.500 over again on the ground. Yeah, and nobody arranged 00:01:08.500 --> 00:01:10.659 those leaves to do that. You know, it happens 00:01:10.659 --> 00:01:13.900 purely because of a phenomenon known as the pinhole 00:01:13.900 --> 00:01:16.359 camera effect. OK, so break that down for us. 00:01:16.379 --> 00:01:18.060 For anyone who hasn't encountered that term before, 00:01:18.159 --> 00:01:20.760 what is a pinhole camera? Well, it's a very simple 00:01:20.760 --> 00:01:23.319 optical trick, really. When light passes through 00:01:23.319 --> 00:01:27.060 a remarkably small opening. like the tiny gaps 00:01:27.060 --> 00:01:29.840 between overlapping leaves, that opening actually 00:01:29.840 --> 00:01:33.180 acts like a simple optical lens. Wow. Yeah. It 00:01:33.180 --> 00:01:35.099 takes the light from the source, which in this 00:01:35.099 --> 00:01:38.299 case is the sun, and faithfully projects an inverted 00:01:38.299 --> 00:01:41.120 image of that source onto whatever surface happens 00:01:41.120 --> 00:01:43.939 to be behind it. So the ground. So instead of 00:01:43.939 --> 00:01:46.920 just letting a generic, you know. blob of light 00:01:46.920 --> 00:01:50.379 through, the leaves are literally acting as thousands 00:01:50.379 --> 00:01:53.599 of tiny natural movie projectors. That's a great 00:01:53.599 --> 00:01:55.500 way to put it. Yeah. They're beaming the live 00:01:55.500 --> 00:01:58.019 status of the sun directly onto the dirt. Which 00:01:58.019 --> 00:02:00.239 is really the perfect entry point for what we 00:02:00.239 --> 00:02:02.400 are exploring today, because an eclipse itself 00:02:02.400 --> 00:02:05.239 is, I mean, it's really just a much grander version 00:02:05.239 --> 00:02:08.340 of that exact same basic idea. It really is. 00:02:08.479 --> 00:02:10.719 And the goal for this deep dive is to take an 00:02:10.719 --> 00:02:14.680 unhurried walk through the mathematically precise 00:02:14.680 --> 00:02:17.560 shadow play of eclipses. I love that. Shadow 00:02:17.560 --> 00:02:19.639 play. Yeah. We're going to explore the geometry 00:02:19.639 --> 00:02:22.039 that makes them possible, the history of how 00:02:22.039 --> 00:02:25.020 early humans figured out the patterns, and just 00:02:25.020 --> 00:02:28.210 the sheer visceral mechanics. of these cosmic 00:02:28.210 --> 00:02:30.310 alignments. Well, to really understand that grand 00:02:30.310 --> 00:02:33.129 shadow play, we have to zoom out from that tree 00:02:33.129 --> 00:02:34.849 for a second and look at the solar system as 00:02:34.849 --> 00:02:37.629 a whole. Okay, zooming out. An eclipse, fundamentally, 00:02:37.889 --> 00:02:41.289 is just one object in space blocking light on 00:02:41.289 --> 00:02:44.370 its way to another object. It's casting a shadow 00:02:44.370 --> 00:02:46.750 shaped by very simple geometry. Right, and there 00:02:46.750 --> 00:02:49.009 are two main types, solar and lunar. And the 00:02:49.009 --> 00:02:51.030 difference basically comes down to which object 00:02:51.030 --> 00:02:53.490 is doing the blocking. Exactly. So in a solar 00:02:53.490 --> 00:02:56.289 eclipse, the moon steps between the sun and earth. 00:02:56.669 --> 00:02:58.870 The moon's shadow falls on us. But in a lunar 00:02:58.870 --> 00:03:01.509 eclipse, Earth steps between the sun and the 00:03:01.509 --> 00:03:04.310 moon, and our massive shadow falls onto the lunar 00:03:04.310 --> 00:03:07.509 surface. Right. But if we start with solar eclipses, 00:03:07.509 --> 00:03:10.430 there is this fundamental question of scale that 00:03:10.430 --> 00:03:12.430 always kind of blows my mind. But the perfect 00:03:12.430 --> 00:03:15.389 fit. Yes. When you look up during a total solar 00:03:15.389 --> 00:03:18.650 eclipse, the moon seems to perfectly, exactly 00:03:18.650 --> 00:03:21.710 cover the sun. It's not too big, not too small. 00:03:21.889 --> 00:03:23.810 Does the math just happen to work out that they 00:03:23.810 --> 00:03:26.569 are physically the same size? Oh, not even close. 00:03:26.650 --> 00:03:29.370 The sun is massive. I mean, it is roughly 400 00:03:29.370 --> 00:03:33.110 times wider than the moon. 400 times. Yeah, but 00:03:33.110 --> 00:03:35.870 there's this staggering geometric coincidence 00:03:35.870 --> 00:03:39.689 at play here. The sun is also, on average, roughly 00:03:39.689 --> 00:03:42.330 400 times further away from Earth than the moon 00:03:42.330 --> 00:03:44.710 is. Wait, let me make sure I'm visualizing this 00:03:44.710 --> 00:03:47.569 correctly. So it would be like holding a tiny 00:03:47.569 --> 00:03:50.210 little coin right at arm's length and realizing 00:03:50.210 --> 00:03:53.550 that this tiny coin perfectly covers up a massive 00:03:53.550 --> 00:03:55.270 dinner plate that somebody's holding all the 00:03:55.270 --> 00:03:57.189 way on the other side of a long room. That is 00:03:57.189 --> 00:04:00.310 precisely how it works. When you multiply those 00:04:00.310 --> 00:04:03.090 two facts together, the 400 times width and the 00:04:03.090 --> 00:04:06.050 400 times distance, you get this extraordinary 00:04:06.050 --> 00:04:08.530 visual trick. It's like an optical illusion built 00:04:08.530 --> 00:04:11.289 into the solar system. Exactly. Seen from our 00:04:11.289 --> 00:04:13.509 vantage point here on Earth, The sun and the 00:04:13.509 --> 00:04:16.250 moon appear to be almost exactly the same size 00:04:16.250 --> 00:04:18.930 in the sky. They are so perfectly matched that 00:04:18.930 --> 00:04:21.889 during a total solar eclipse, the moon can precisely 00:04:21.889 --> 00:04:25.550 cover the sun's bright disk. No more. And no 00:04:25.550 --> 00:04:27.310 less. Which, I mean, that raises the question 00:04:27.310 --> 00:04:30.509 of why. Is there some gravitational law or, I 00:04:30.509 --> 00:04:32.730 don't know, physics rule that mandates this exact 00:04:32.730 --> 00:04:35.389 ratio in planetary systems? Yeah, and that's 00:04:35.389 --> 00:04:37.430 the wild part. As far as anyone in astronomy 00:04:37.430 --> 00:04:40.149 or astrophysics can tell, there is absolutely 00:04:40.149 --> 00:04:43.529 no known physical reason why those two objects 00:04:43.529 --> 00:04:45.670 have to line up this neatly. Wait, really? It's 00:04:45.670 --> 00:04:47.750 just random. It is simply a pure coincidence 00:04:47.750 --> 00:04:50.490 of how our particular solar system happened to 00:04:50.490 --> 00:04:53.699 form. That is wild. Well, this perfect match 00:04:53.699 --> 00:04:55.800 isn't even a permanent fixture. Because the moon 00:04:55.800 --> 00:04:58.220 isn't stationary, right? Like it's slowly drifting. 00:04:58.500 --> 00:05:01.480 Exactly. The moon is gradually moving away from 00:05:01.480 --> 00:05:03.819 Earth over these extremely long time scales, 00:05:03.980 --> 00:05:07.399 which means that total eclipses, as we experience 00:05:07.399 --> 00:05:10.259 them right now, are a temporary feature. Wow. 00:05:10.459 --> 00:05:14.240 Yeah. In the very long run of our planet's multi 00:05:14.240 --> 00:05:17.259 -billion year history, this... Perfect alignment 00:05:17.259 --> 00:05:20.459 is just a passing phase. We just happen to exist 00:05:20.459 --> 00:05:22.920 during the exact right window of time to see 00:05:22.920 --> 00:05:25.779 it. Okay, so we have this perfect temporary alignment. 00:05:26.540 --> 00:05:28.879 But the moon is orbiting the Earth, completing 00:05:28.879 --> 00:05:31.600 a lap roughly once a month, right? Correct. And 00:05:31.600 --> 00:05:34.100 it passes between us and the sun during every 00:05:34.100 --> 00:05:37.759 single new moon phase. Right, exactly. So if 00:05:37.759 --> 00:05:39.339 it's passing between us and the sun every single 00:05:39.339 --> 00:05:41.560 month, why aren't we plunged into darkness every 00:05:41.560 --> 00:05:43.639 30 days? Right, that's the obvious question. 00:05:43.720 --> 00:05:45.339 And it all comes down to what you could call 00:05:45.339 --> 00:05:47.660 orbital fussiness. Orbital fussiness, I like 00:05:47.660 --> 00:05:49.959 that. The key is that the moon's orbit is not 00:05:49.959 --> 00:05:52.579 perfectly flat relative to Earth's orbit. Okay. 00:05:53.019 --> 00:05:54.860 If you imagine the flat plane that the Earth 00:05:54.860 --> 00:05:58.860 travels on as it orbits the sun. Astronomers 00:05:58.860 --> 00:06:00.819 call this flat plane the ecliptic. Ecliptic, 00:06:00.819 --> 00:06:03.600 right. Yeah. So the moon's orbit around Earth 00:06:03.600 --> 00:06:06.060 doesn't sit perfectly level on that ecliptic. 00:06:06.160 --> 00:06:08.339 It is actually tilted by about five degrees. 00:06:08.579 --> 00:06:13.040 Okay, so if I picture like two hula hoops, one 00:06:13.040 --> 00:06:15.199 inside the other. Like an analogy. The outer 00:06:15.199 --> 00:06:17.379 hula hoop is the Earth going around the sun. 00:06:17.800 --> 00:06:20.240 And the inner hula hoop is the moon going around 00:06:20.240 --> 00:06:22.560 the Earth. But the inner one is just slightly 00:06:22.560 --> 00:06:25.519 tipped. Yes. So most of the time, the edges just 00:06:25.519 --> 00:06:27.899 don't actually line up. Exactly. I mean, five 00:06:27.899 --> 00:06:30.079 degrees sounds pretty negligible, right? But 00:06:30.079 --> 00:06:32.800 in the vastness of space, it is huge. Right, 00:06:32.879 --> 00:06:35.600 because of the distance. Exactly. It's more than 00:06:35.600 --> 00:06:38.259 enough that during most months, when the new 00:06:38.259 --> 00:06:40.879 moon passes in front of the sun, from our perspective, 00:06:41.139 --> 00:06:43.870 it actually passes slightly above the sun. you 00:06:43.870 --> 00:06:45.649 know slightly below it so it just misses the 00:06:45.649 --> 00:06:48.410 mark completely yeah his shadow gets cast out 00:06:48.410 --> 00:06:51.269 into empty space completely missing earth what 00:06:51.269 --> 00:06:53.089 actually has to happen for the shadow to hit 00:06:53.089 --> 00:06:56.350 us then well for an eclipse to occur a new moon 00:06:56.350 --> 00:06:58.290 has to happen very close to what astronomers 00:06:58.290 --> 00:07:02.430 call nodes yeah nodes these are the exact specific 00:07:02.430 --> 00:07:05.829 intersection points where that tilted lunar orbit 00:07:06.220 --> 00:07:08.439 physically crosses the flat plane of the Earth's 00:07:08.439 --> 00:07:10.779 orbit. So you need the moon to be at the new 00:07:10.779 --> 00:07:13.060 moon phase, and you need it to be sitting right 00:07:13.060 --> 00:07:16.019 at one of those precise intersection nodes. That's 00:07:16.019 --> 00:07:19.379 why eclipses are these rare, geometrically fussy 00:07:19.379 --> 00:07:21.860 events, and not just a monthly routine. Right. 00:07:21.980 --> 00:07:24.319 But even when they do hit those nodes and cast 00:07:24.319 --> 00:07:26.199 a shadow on Earth, the shadow doesn't always 00:07:26.199 --> 00:07:28.220 look the same, right? Like, we don't always get 00:07:28.220 --> 00:07:30.420 a total eclipse. Right, exactly. Sometimes we 00:07:30.420 --> 00:07:33.500 get an annular eclipse or a partial one. But 00:07:33.500 --> 00:07:36.139 if the ratio of the sun and moon size is so perfect, 00:07:36.199 --> 00:07:38.879 like we just talked about, what causes those 00:07:38.879 --> 00:07:41.259 variations? That comes down to another slight 00:07:41.259 --> 00:07:43.399 imperfection in the system. The moon's orbit 00:07:43.399 --> 00:07:46.839 isn't a perfect circle. It is slightly elliptical, 00:07:46.879 --> 00:07:50.259 meaning it's a bit oval -shaped. Oh, I see. So 00:07:50.259 --> 00:07:51.800 the physical distance between the Earth and the 00:07:51.800 --> 00:07:53.519 moon actually changes a bit throughout the month. 00:07:53.980 --> 00:07:56.300 When the moon happens to be at its closest point 00:07:56.300 --> 00:07:58.139 to Earth during one of those node alignments, 00:07:58.259 --> 00:08:01.449 it appears very slightly larger in our sky. And 00:08:01.449 --> 00:08:04.170 that's what gives us a total solar eclipse. Yes. 00:08:04.449 --> 00:08:08.350 The moon completely covers the sun's disk, plunging 00:08:08.350 --> 00:08:11.110 the area beneath the shadow into that eerie, 00:08:11.170 --> 00:08:13.990 temporary daytime darkness. But if the moon happens 00:08:13.990 --> 00:08:16.910 to be on the further end of that oval orbit when 00:08:16.910 --> 00:08:19.509 it crosses the node, then it appears very slightly 00:08:19.509 --> 00:08:22.579 smaller in the sky. So it isn't quite big enough 00:08:22.579 --> 00:08:24.899 to cover the whole solar disk. Right. And this 00:08:24.899 --> 00:08:27.300 is what creates an annular eclipse. People often 00:08:27.300 --> 00:08:29.699 call it a ring of fire. Oh, yeah, the ring of 00:08:29.699 --> 00:08:32.240 fire. Those pictures are incredible. They really 00:08:32.240 --> 00:08:36.360 are. Because a brilliant, thin ring of actual 00:08:36.360 --> 00:08:39.519 sunlight remains visible around the dark silhouette 00:08:39.519 --> 00:08:42.419 of the moon the entire time. And a partial eclipse 00:08:42.419 --> 00:08:44.879 would just be when the alignment isn't perfectly 00:08:44.879 --> 00:08:47.970 dead center? Exactly. Or maybe the observer on 00:08:47.970 --> 00:08:49.909 Earth is standing off to the side, you know, 00:08:49.909 --> 00:08:51.870 in the outer weaker part of the moon shadow. 00:08:52.450 --> 00:08:55.070 The moon only covers a chunk of the sun. Which 00:08:55.070 --> 00:08:57.090 brings us right back to our opening under the 00:08:57.090 --> 00:08:59.470 tree, producing all those little crescent shapes 00:08:59.470 --> 00:09:01.759 on the footpath. Full circle. Yeah, it's all 00:09:01.759 --> 00:09:04.179 dictated by distance and angles. And because 00:09:04.179 --> 00:09:07.179 this geometry operates basically like clockwork, 00:09:07.360 --> 00:09:10.100 it is incredibly predictable once you actually 00:09:10.100 --> 00:09:12.379 know the mechanics. Right. But early humans didn't 00:09:12.379 --> 00:09:14.440 know the mechanics. No, they definitely did not. 00:09:14.519 --> 00:09:16.539 I mean, before we understood elliptical orbits 00:09:16.539 --> 00:09:18.700 and five degree tilts, an eclipse must have been 00:09:18.700 --> 00:09:21.960 deeply, deeply unsettling. Just imagine you're 00:09:21.960 --> 00:09:24.539 going about your day and the sun just gets eaten 00:09:24.539 --> 00:09:27.419 by a black void. It would be terrifying. There's 00:09:27.419 --> 00:09:29.820 actually an old Chinese legend about two court 00:09:29.820 --> 00:09:32.980 astronomers, Hai and Hou, and they were supposedly 00:09:32.980 --> 00:09:35.639 executed for failing to predict an eclipse and 00:09:35.639 --> 00:09:39.039 warn the emperor in time. Oh, wow. High stakes. 00:09:39.320 --> 00:09:42.299 Right. And whether that's folklore or hard history, 00:09:42.460 --> 00:09:44.779 it really illustrates how early societies viewed 00:09:44.779 --> 00:09:47.480 these events. They weren't fun science demonstrations. 00:09:47.740 --> 00:09:50.879 They were terrifying omens that demanded an urgent 00:09:50.879 --> 00:09:53.659 official response. And because ancient astronomers 00:09:53.659 --> 00:09:55.960 were so heavily motivated to see these omens 00:09:55.960 --> 00:09:58.559 coming, They started noticing patterns. Because 00:09:58.559 --> 00:10:01.039 their lives literally depended on it. Exactly. 00:10:01.299 --> 00:10:03.399 Even without knowing the underlying physics, 00:10:03.600 --> 00:10:06.259 they realized eclipses weren't entirely random. 00:10:06.440 --> 00:10:09.340 And this led to the discovery of the Saros Cycle. 00:10:09.440 --> 00:10:11.879 Okay, the Saros Cycle. How does that cycle actually 00:10:11.879 --> 00:10:15.080 work? It is a marvel of early human observation. 00:10:15.220 --> 00:10:18.720 It is a period of very close to 18 years, 11 00:10:18.720 --> 00:10:22.860 days, and 8 hours. 18 years, 11 days, and 8 hours. 00:10:22.899 --> 00:10:25.799 That is very specific. Extremely specific. And 00:10:25.799 --> 00:10:28.580 after that exact amount of time, the sun, the 00:10:28.580 --> 00:10:31.419 earth, and the moon return to a nearly identical 00:10:31.419 --> 00:10:35.419 relative geometric arrangement. Because of this, 00:10:35.539 --> 00:10:38.879 a very similar eclipse will recur. But the extra 00:10:38.879 --> 00:10:40.879 8 hours is the catch, though, right? Because 00:10:40.879 --> 00:10:43.909 8 hours is a third of a day. meaning the Earth 00:10:43.909 --> 00:10:46.049 has rotated a third of the way around its axis 00:10:46.049 --> 00:10:47.970 since the last eclipse. And you nailed it. So 00:10:47.970 --> 00:10:49.610 the shadow will fall on a completely different 00:10:49.610 --> 00:10:52.309 part of the globe. Exactly right. And the reason 00:10:52.309 --> 00:10:54.929 the Saro cycle works at all is because it represents 00:10:54.929 --> 00:10:58.190 the precise moment when three completely separate 00:10:58.190 --> 00:11:01.230 lunar rhythms happen to sync up simultaneously. 00:11:01.629 --> 00:11:04.269 Wow, okay. Yeah. You have the time it takes the 00:11:04.269 --> 00:11:06.370 moon to return to the same phase. You have the 00:11:06.370 --> 00:11:07.889 time it takes the moon to return to the same 00:11:07.889 --> 00:11:10.690 node, you know, that intersection point in its 00:11:10.690 --> 00:11:12.509 tilted orbit. And you have the time it takes 00:11:12.509 --> 00:11:14.350 the moon to return to the same distance from 00:11:14.350 --> 00:11:16.990 Earth in its oval -shaped orbit. So it's like 00:11:16.990 --> 00:11:19.610 having three different ticking clocks, all with 00:11:19.610 --> 00:11:21.690 slightly different lengths, and just waiting 00:11:21.690 --> 00:11:24.149 for the exact moment they all chime at the same 00:11:24.149 --> 00:11:26.509 second. That is a perfect analogy. What stands 00:11:26.509 --> 00:11:29.490 out to me is that Babylonian, Chinese, and Greek 00:11:29.490 --> 00:11:32.139 observers figured this out through... Through 00:11:32.139 --> 00:11:35.460 sheer brute force pattern spotting. It's incredible. 00:11:35.759 --> 00:11:39.480 They kept meticulous records over centuries just 00:11:39.480 --> 00:11:42.159 to predict these events, long before they understood 00:11:42.159 --> 00:11:45.139 gravity or orbits. Though, you know, we should 00:11:45.139 --> 00:11:47.399 give credit where it's due. The Greek philosopher 00:11:47.399 --> 00:11:50.559 Anaxagoras actually did correctly guess the geometry 00:11:50.559 --> 00:11:53.720 way back in the 5th century BCE. Wait, really? 00:11:53.899 --> 00:11:57.700 The 5th century BCE? Yeah. He argued that a solar 00:11:57.700 --> 00:12:00.240 eclipse was just the moon passing in front of 00:12:00.240 --> 00:12:02.860 the sun. It was a strikingly modern explanation 00:12:02.860 --> 00:12:05.700 for an ancient world that still largely relied 00:12:05.700 --> 00:12:09.330 on mythology. That is amazing. But if Anaxagoras 00:12:09.330 --> 00:12:11.909 was the start of understanding eclipses, the 00:12:11.909 --> 00:12:14.730 absolute pinnacle of eclipse science has to be 00:12:14.730 --> 00:12:17.529 1919. Oh, absolutely. Because this is the moment 00:12:17.529 --> 00:12:19.769 a shadow literally changed how we understand 00:12:19.769 --> 00:12:21.149 the fabric of the universe. You're referring 00:12:21.149 --> 00:12:23.429 to Arthur Eddington's expedition. Yes. In May 00:12:23.429 --> 00:12:26.169 of 1919, the British astronomer Arthur Eddington 00:12:26.169 --> 00:12:29.230 led teams to the island of Principe off West 00:12:29.230 --> 00:12:32.509 Africa and to Sobral in Brazil, specifically 00:12:32.509 --> 00:12:35.629 to observe a total solar eclipse. But he wasn't 00:12:35.629 --> 00:12:37.350 there to study the eclipse itself, though, right? 00:12:37.389 --> 00:12:40.230 No, not at all. He was using the eclipse as a 00:12:40.230 --> 00:12:43.149 tool to test a brand new, highly controversial 00:12:43.149 --> 00:12:45.649 theory published by Albert Einstein. General 00:12:45.649 --> 00:12:48.929 relativity. Precisely. Einstein's theory proposed 00:12:48.929 --> 00:12:52.200 something totally radical. that massy objects 00:12:52.200 --> 00:12:55.559 like our sun actually bend the physical fabric 00:12:55.559 --> 00:12:57.679 of space and time around them. Which sounds completely 00:12:57.679 --> 00:13:00.539 wild. It sounded absurd to a lot of people. But 00:13:00.539 --> 00:13:02.639 because of that bent space, Einstein predicted 00:13:02.639 --> 00:13:04.980 that light traveling from distant stars should 00:13:04.980 --> 00:13:08.620 be deflected or bent very slightly as it passed 00:13:08.620 --> 00:13:11.059 close to the heavy mass of the sun on its way 00:13:11.059 --> 00:13:13.840 to our eyes here on Earth. Right, but the problem 00:13:13.840 --> 00:13:17.279 with testing that... on a normal tuesday is that 00:13:17.279 --> 00:13:19.200 you can't just look at stars right next to the 00:13:19.200 --> 00:13:22.399 sun no definitely not the sun's glare is blinding 00:13:22.399 --> 00:13:25.679 the sky is bright blue and the stars are completely 00:13:25.679 --> 00:13:28.539 invisible which is why totality is so crucial 00:13:28.539 --> 00:13:31.139 for this during those few minutes of a total 00:13:31.139 --> 00:13:34.620 solar eclipse the sun's bright disk is completely 00:13:34.620 --> 00:13:38.730 blocked by the moon the sky goes dark suddenly 00:13:38.730 --> 00:13:41.450 stars close to the sun's position in the sky 00:13:41.450 --> 00:13:44.230 become briefly visible right in the middle of 00:13:44.230 --> 00:13:46.110 the day. It's like turning off the lights in 00:13:46.110 --> 00:13:48.250 a room so you can see out the window. Exactly. 00:13:48.330 --> 00:13:50.450 So Eddington's team photographed those stars 00:13:50.450 --> 00:13:53.570 during the eclipse. Then they compared those 00:13:53.570 --> 00:13:56.149 photos against photos of the exact same stars 00:13:56.149 --> 00:13:59.149 taken months earlier at night when the sun was 00:13:59.149 --> 00:14:01.070 nowhere near them. And when they developed those 00:14:01.070 --> 00:14:03.669 photos, the stars in the eclipse image appeared 00:14:03.669 --> 00:14:06.029 to have shifted. Yes. They had moved by almost 00:14:06.029 --> 00:14:08.629 the exact amount that Einstein's math predicted. 00:14:08.809 --> 00:14:11.450 The sheer mass of the sun had bent the light 00:14:11.450 --> 00:14:14.029 passing by it. It's just phenomenal. A shadow 00:14:14.029 --> 00:14:16.629 lasting just a few minutes provided the perfect 00:14:16.629 --> 00:14:19.169 natural laboratory to prove that space -time 00:14:19.169 --> 00:14:21.740 is flexible. I mean... It established relativity 00:14:21.740 --> 00:14:24.639 as serious, tested physics. It is a beautiful 00:14:24.639 --> 00:14:27.299 convergence of events. Yeah. It really highlights 00:14:27.299 --> 00:14:30.659 just how uniquely valuable totality is for science. 00:14:30.879 --> 00:14:33.379 And beyond the science, the experience of totality 00:14:33.379 --> 00:14:37.100 itself is described as just a genuinely startling, 00:14:37.100 --> 00:14:40.600 bizarre sensory event. Oh, it is. It's profoundly 00:14:40.600 --> 00:14:43.039 weird. It's not just that the sky gets a bit 00:14:43.039 --> 00:14:46.039 dark, right? In the minutes leading up to totality, 00:14:46.139 --> 00:14:48.580 the quality of the light fundamentally changes. 00:14:48.799 --> 00:14:51.639 It flattens out. Yes, it flattens. Shadows sharpen 00:14:51.639 --> 00:14:53.899 in a way that feels incredibly unnatural. You 00:14:53.899 --> 00:14:55.600 can actually feel the temperature measurably 00:14:55.600 --> 00:14:58.279 drop as the sunlight is suddenly cut off. And 00:14:58.279 --> 00:15:01.240 then for anywhere from a few seconds to maybe 00:15:01.240 --> 00:15:04.139 seven minutes, totality sets in. And to understand 00:15:04.139 --> 00:15:06.019 what you're seeing in that specific moment, we 00:15:06.019 --> 00:15:08.059 need to define two distinct parts of the sun. 00:15:08.240 --> 00:15:11.120 Okay. First, there's the photosphere. That is 00:15:11.120 --> 00:15:14.639 the sun's bright, normally visible disk. It's 00:15:14.639 --> 00:15:16.779 the blinding surface we're used to seeing. Right, 00:15:16.879 --> 00:15:19.250 the part that hurts to look at. Exactly. During 00:15:19.250 --> 00:15:21.610 totality, that photosphere is completely blocked. 00:15:21.830 --> 00:15:24.250 And because it's blocked, something normally 00:15:24.250 --> 00:15:28.100 invisible is unveiled. The corona. The corona 00:15:28.100 --> 00:15:30.539 being the sun's atmosphere. Right. It is this 00:15:30.539 --> 00:15:34.120 wispy pale halo of extremely hot, extremely thin 00:15:34.120 --> 00:15:37.460 plasma that extends millions of kilometers out 00:15:37.460 --> 00:15:39.539 into space from the sun's surface. And it's always 00:15:39.539 --> 00:15:41.820 there, isn't it? It is always there. Under normal 00:15:41.820 --> 00:15:43.440 daylight conditions, it's happening right now. 00:15:43.559 --> 00:15:46.720 But it is roughly a million times dimmer than 00:15:46.720 --> 00:15:49.799 the photosphere. It is just... utterly overwhelmed 00:15:49.799 --> 00:15:53.039 by the glare so totality removes the glare and 00:15:53.039 --> 00:15:56.379 the corona appears exactly and for a huge chunk 00:15:56.379 --> 00:15:59.200 of the 20th century a total eclipse was basically 00:15:59.200 --> 00:16:02.039 the only practical way for scientists to study 00:16:02.039 --> 00:16:04.700 the corona at all it's that dramatic reveal that 00:16:04.700 --> 00:16:06.860 drives you know eclipse chasers to travel the 00:16:06.860 --> 00:16:09.480 globe heading to isolated places like xmith in 00:16:09.480 --> 00:16:12.700 western australia in 2023 or like preparing for 00:16:12.700 --> 00:16:15.799 sydney in july 2028 oh yeah they plan years in 00:16:15.799 --> 00:16:18.139 advance they are chasing these fleeting visual 00:16:18.250 --> 00:16:20.210 phenomena that only happen for seconds, like 00:16:20.210 --> 00:16:22.970 Bailey's beads. Oh, Bailey's beads are spectacular. 00:16:23.370 --> 00:16:26.070 They occur just before and just after totality. 00:16:26.190 --> 00:16:28.610 What causes them? Well, the moon doesn't have 00:16:28.610 --> 00:16:30.950 a perfectly smooth edge, right? It is covered 00:16:30.950 --> 00:16:33.830 in mountains and deep valleys. Oh, right. So 00:16:33.830 --> 00:16:36.309 right at the edge of alignment, sunlight streams 00:16:36.309 --> 00:16:39.049 through those deep lunar valleys, breaking up 00:16:39.049 --> 00:16:41.409 into a scatter of brilliant, distinct points 00:16:41.409 --> 00:16:44.909 of light around the moon's silhouette. It looks 00:16:44.909 --> 00:16:47.840 like a string of glowing lamps. And then as the 00:16:47.840 --> 00:16:51.500 very last bead fades right before totality, or 00:16:51.500 --> 00:16:54.259 as the very first one reappears after, you get 00:16:54.259 --> 00:16:56.220 what is called the diamond ring effect. Yes, 00:16:56.240 --> 00:16:59.710 the classic diamond ring. It is one single intense 00:16:59.710 --> 00:17:02.909 glowing point of light attached to this pale 00:17:02.909 --> 00:17:05.829 ring. It is arguably the most photographed moment 00:17:05.829 --> 00:17:08.250 of any eclipse. Oh, without a doubt. And while 00:17:08.250 --> 00:17:09.970 you're watching this, birds are suddenly falling 00:17:09.970 --> 00:17:12.609 silent, insects start chirping thinking it's 00:17:12.609 --> 00:17:14.990 night, and you might even see shadow bands. Shadow 00:17:14.990 --> 00:17:17.029 bands are such a strange phenomenon. They are 00:17:17.029 --> 00:17:19.670 these fast -moving, rippling patterns of light 00:17:19.670 --> 00:17:22.950 and shadow that can race across pale, flat surfaces 00:17:22.950 --> 00:17:25.809 right around totality. Like ripples at the bottom 00:17:25.809 --> 00:17:29.430 of a pool. Yes. Exactly like that. They're caused 00:17:29.430 --> 00:17:32.230 by that tiny remaining sliver of sunlight being 00:17:32.230 --> 00:17:35.130 refracted unevenly by turbulence in Earth's own 00:17:35.130 --> 00:17:38.369 atmosphere. That is so cool. Okay, so if the 00:17:38.369 --> 00:17:40.690 moon's shadow causes all this daytime drama, 00:17:40.950 --> 00:17:43.869 what happens when we reverse the geometry? Let's 00:17:43.869 --> 00:17:46.470 talk about lunar eclipses. The lunar reverse. 00:17:46.589 --> 00:17:49.269 Right. Earth steps into the middle, and it's 00:17:49.269 --> 00:17:51.650 our massive shadow blocking the sunlight from 00:17:51.650 --> 00:17:53.609 hitting the full moon. The immediate question 00:17:53.609 --> 00:17:56.539 is... If the Earth is completely blocking the 00:17:56.539 --> 00:17:59.079 sun, why doesn't the moon just go pitch black? 00:17:59.319 --> 00:18:01.119 That's a great question. Why does it turn that 00:18:01.119 --> 00:18:04.339 famous deep coppery red, you know, the blood 00:18:04.339 --> 00:18:07.559 moon? The answer to that lies in our own planet's 00:18:07.559 --> 00:18:09.779 atmosphere through a process called Rayleigh 00:18:09.779 --> 00:18:12.519 scattering. Okay, walk us through how Rayleigh 00:18:12.519 --> 00:18:15.599 scattering turns a dark shadow red. Sure. So 00:18:15.599 --> 00:18:17.680 as sunlight travels toward Earth, it hits our 00:18:17.680 --> 00:18:19.960 atmosphere. And our atmosphere acts like a lens 00:18:19.960 --> 00:18:22.559 and a filter combined. It bends some of that 00:18:22.559 --> 00:18:25.119 light inward. toward the shadow behind Earth. 00:18:25.500 --> 00:18:28.359 But as it does this, it filters out the shorter, 00:18:28.500 --> 00:18:31.680 bluer wavelengths of light. This exact scattering 00:18:31.680 --> 00:18:33.980 of blue light is actually the reason our daytime 00:18:33.980 --> 00:18:36.720 sky looks blue to us down on the ground. Because 00:18:36.720 --> 00:18:38.859 the blue light gets trapped and scattered around 00:18:38.859 --> 00:18:41.480 in the atmosphere. Exactly. So if the atmosphere 00:18:41.480 --> 00:18:44.119 scatters away all the blue light, what's left 00:18:44.119 --> 00:18:46.519 to pass through? The longer wavelengths. The 00:18:46.519 --> 00:18:49.759 reds and oranges. Yes. The reds, the oranges. 00:18:50.490 --> 00:18:53.369 The coppery colors. This reddish light survives 00:18:53.369 --> 00:18:55.450 the trip through the edges of Earth's atmosphere, 00:18:55.650 --> 00:18:58.369 gets bent into our shadow by refraction, and 00:18:58.369 --> 00:19:00.710 falls squarely onto the moon's surface. So the 00:19:00.710 --> 00:19:02.650 Earth's atmosphere is basically acting like a 00:19:02.650 --> 00:19:05.630 giant cosmic color filter. Pretty much. It tosses 00:19:05.630 --> 00:19:07.650 out the blue light and it bends the reddish light 00:19:07.650 --> 00:19:09.759 from the edges of the Earth. It's projecting 00:19:09.759 --> 00:19:11.940 the collective light of all the world's sunsets 00:19:11.940 --> 00:19:14.059 and sunrises onto the moon at the exact same 00:19:14.059 --> 00:19:16.799 time. That is an incredibly accurate way to describe 00:19:16.799 --> 00:19:19.380 it. A blood moon is quite literally illuminated 00:19:19.380 --> 00:19:22.319 by every sunset and sunrise happening on Earth 00:19:22.319 --> 00:19:24.900 in that specific moment. That is almost poetic. 00:19:25.119 --> 00:19:27.779 It really is. And what makes lunar eclipses so 00:19:27.779 --> 00:19:30.920 accessible is their scale. Earth's shadow is 00:19:30.920 --> 00:19:34.170 massive compared to the moon. Right. So when 00:19:34.170 --> 00:19:36.630 a lunar eclipse happens, you don't have to travel 00:19:36.630 --> 00:19:39.369 to a tiny specific path like you do for a total 00:19:39.369 --> 00:19:42.230 solar eclipse. A lunar eclipse is visible to 00:19:42.230 --> 00:19:45.089 the entire nighttime hemisphere of Earth simultaneously. 00:19:45.849 --> 00:19:48.990 And it lasts for hours, not minutes. You literally 00:19:48.990 --> 00:19:51.049 just walk outside and look up, which actually 00:19:51.049 --> 00:19:53.490 brings us to a really critical safety point here. 00:19:53.589 --> 00:19:55.589 Yes, very important. Looking at a lunar eclipse 00:19:55.589 --> 00:19:58.509 is perfectly safe. Looking at a solar eclipse, 00:19:58.670 --> 00:20:01.710 however, requires understanding a very real biological 00:20:01.710 --> 00:20:04.049 danger. We have to be very direct about this. 00:20:04.269 --> 00:20:07.069 Looking directly at the sun at any point during 00:20:07.069 --> 00:20:09.809 a partial solar eclipse or even during the partial 00:20:09.809 --> 00:20:12.829 phases leading up to and following totality is 00:20:12.829 --> 00:20:15.230 intensely dangerous to your eyesight. Even if 00:20:15.230 --> 00:20:17.670 the sun looks dimmer or a large chunk of it is 00:20:17.670 --> 00:20:20.430 missing? the invisible radiation is still hitting 00:20:20.430 --> 00:20:23.250 your eye. And the scariest part is that you won't 00:20:23.250 --> 00:20:25.750 feel it happening. No, you won't. The retina, 00:20:25.750 --> 00:20:27.630 which is the light -sensitive tissue at the back 00:20:27.630 --> 00:20:29.589 of your eye, has absolutely no pain receptors. 00:20:29.809 --> 00:20:32.230 None. So you just don't feel it burning. Exactly. 00:20:32.289 --> 00:20:34.869 This means you can literally burn permanent blind 00:20:34.869 --> 00:20:37.849 spots into your vision without feeling a single 00:20:37.849 --> 00:20:40.349 thing while the damage is occurring. The damage 00:20:40.349 --> 00:20:42.950 is silent and it is permanent. And regular sunglasses 00:20:42.950 --> 00:20:45.910 don't help, right? Ordinary sunglasses, regardless 00:20:45.910 --> 00:20:48.670 of how dark or how expensive they are, offer 00:20:48.670 --> 00:20:52.069 zero protection against this specific kind of 00:20:52.069 --> 00:20:55.829 solar radiation. So to view a partial solar eclipse 00:20:55.829 --> 00:20:59.369 safely... You must use purpose -built certified 00:20:59.369 --> 00:21:02.150 solar filters, you know, the specific cardboard 00:21:02.150 --> 00:21:04.410 eclipse glasses with international safety ratings. 00:21:04.589 --> 00:21:06.950 Yes. Or you go back to where we started this 00:21:06.950 --> 00:21:09.869 deep dive, indirect viewing. You make a pinhole 00:21:09.869 --> 00:21:11.890 projector with a piece of card and you safely 00:21:11.890 --> 00:21:14.769 watch the projection on the ground, just like 00:21:14.769 --> 00:21:18.289 the leaves do. There is one and only one genuine 00:21:18.289 --> 00:21:20.470 exception to this rule. And that's totality. 00:21:20.630 --> 00:21:25.190 Yes. The brief window of 100 % totality. Only 00:21:25.190 --> 00:21:28.250 when the sun's bright disk is completely, totally 00:21:28.250 --> 00:21:31.190 covered by the moon does it become safe and truly 00:21:31.190 --> 00:21:33.849 spectacular to look directly at the Carina with 00:21:33.849 --> 00:21:35.869 the naked eye. But you have to be absolutely 00:21:35.869 --> 00:21:39.210 certain totality has begun. Exactly. And the 00:21:39.210 --> 00:21:41.549 second that diamond ring flashes and the sun 00:21:41.549 --> 00:21:44.769 starts to reappear, the protective filters must 00:21:44.769 --> 00:21:47.680 go straight back on. It requires care, but the 00:21:47.680 --> 00:21:49.900 payoff of seeing the corona is apparently just 00:21:49.900 --> 00:21:53.279 life -changing. It's unparalleled. So, as we 00:21:53.279 --> 00:21:55.180 wrap up our journey through the shadows today, 00:21:55.319 --> 00:21:57.640 I want to bring you back to that warm afternoon. 00:21:57.859 --> 00:22:00.859 Put yourself back under that leafy tree. Right 00:22:00.859 --> 00:22:02.900 back where we started. The next time you find 00:22:02.900 --> 00:22:05.299 yourself standing under a canopy on an ordinary 00:22:05.299 --> 00:22:08.599 sunny afternoon, remember what is actually happening. 00:22:09.200 --> 00:22:12.099 Every single gap in the leaves above you is quietly 00:22:12.099 --> 00:22:15.299 acting as a camera, faithfully tracing the truth 00:22:15.299 --> 00:22:17.599 of the cosmos right onto the ground at your feet, 00:22:17.700 --> 00:22:19.880 whether anything unusual is happening up there 00:22:19.880 --> 00:22:23.059 or not. It is a quiet, ongoing record of our 00:22:23.059 --> 00:22:26.480 solar system's mechanics. But, you know, understanding 00:22:26.480 --> 00:22:29.579 these incredibly precise mechanics opens up one 00:22:29.579 --> 00:22:32.539 final, fascinating reality to think about. Oh, 00:22:32.619 --> 00:22:34.619 what's that? We talked earlier about how the 00:22:34.619 --> 00:22:36.259 moon is gradually moving away from the Earth, 00:22:36.339 --> 00:22:38.299 right? Right, the slow drift. Because of that 00:22:38.299 --> 00:22:41.759 drift, We know that one day, millions of years 00:22:41.759 --> 00:22:44.279 from now, the moon will be too far away to ever 00:22:44.279 --> 00:22:47.559 fully cover the sun's disk. Total eclipses will 00:22:47.559 --> 00:22:50.859 simply cease to exist on Earth. Wow. Future beings 00:22:50.859 --> 00:22:53.740 will only ever see partial or annular eclipses, 00:22:53.740 --> 00:22:55.960 never a total one. That's sort of heartbreaking, 00:22:56.099 --> 00:22:58.599 but also amazing. It really is. And it invites 00:22:58.599 --> 00:23:01.829 a pretty profound question. What other temporary 00:23:01.829 --> 00:23:04.970 cosmic masterpieces are we just taking for granted 00:23:04.970 --> 00:23:07.970 right now, simply because our tiny slice of human 00:23:07.970 --> 00:23:10.269 history happened to align perfectly with the 00:23:10.269 --> 00:23:12.269 clockwork of the universe? Something to mull 00:23:12.269 --> 00:23:13.769 over the next time you're standing in the shade.