WEBVTT 00:00:03.110 --> 00:00:07.970 We humans of planet Earth benefit from a great coincidence. It’s a coincidence of time, 00:00:07.970 --> 00:00:09.720 and of space. And of math. 00:00:09.730 --> 00:00:14.630 The coincidence is this: the Sun is about 400 times wider than the Moon, and it’s 00:00:14.630 --> 00:00:18.320 also on average about 400 times farther away than the Moon. 00:00:18.330 --> 00:00:22.110 The apparent size of an object in the sky depends on how big it is and how far away 00:00:22.110 --> 00:00:26.940 it is... so these numbers being equal means the Sun and the Moon appear to be about the 00:00:26.940 --> 00:00:28.450 same size in the sky. 00:00:28.450 --> 00:00:32.840 And that’s where another interesting thing comes in: Sometimes, the Moon passes directly 00:00:32.840 --> 00:00:38.120 between the Earth and the Sun. It doesn’t happen all that often, but when it does, you get magic. 00:00:38.120 --> 00:00:40.300 Or even better: You get SCIENCE. 00:00:40.300 --> 00:00:41.960 You get an eclipse. 00:00:52.100 --> 00:00:56.620 An eclipse is a generic term in astronomy for when one object passes into the shadow 00:00:56.630 --> 00:00:59.100 of another object, darkening or blocking it. 00:00:59.100 --> 00:01:03.989 A solar eclipse is when the Moon blocks the Sun, casting a shadow on the Earth, and a 00:01:03.989 --> 00:01:08.180 lunar eclipse is when the Earth blocks the Sun, casting a shadow on the Moon. 00:01:08.180 --> 00:01:09.000 But how do they work? 00:01:09.000 --> 00:01:14.250 Well, the Moon orbits the Earth once per month, and the Earth orbits the Sun once per year. 00:01:14.250 --> 00:01:18.030 If the Moon’s orbit were perfectly aligned with the Earth’s, essentially sharing the 00:01:18.030 --> 00:01:23.110 same plane, we’d get a solar eclipse every new Moon and a lunar eclipse every full Moon! 00:01:23.110 --> 00:01:26.650 But we don’t. That’s because the Moon’s orbit is tilted with respect to Earth’s, 00:01:26.650 --> 00:01:28.200 by about 5°. 00:01:28.200 --> 00:01:33.540 What that means is that, at new Moon, the Moon can be as much as 5° away from the Sun, 00:01:33.540 --> 00:01:39.000 passing “above” or “below” the Sun in the sky, thereby missing it, from our perspective. 00:01:39.000 --> 00:01:43.939 But sometimes the Moon is in the right place at the right time, and at new Moon, it lies 00:01:43.939 --> 00:01:48.119 perfectly in line between the Sun and the Earth. And when that happens, we get a solar 00:01:48.120 --> 00:01:53.820 eclipse. This geometry happens at least twice per year, and sometimes as much as five times per year. 00:01:53.820 --> 00:01:58.780 What’s happening physically in space is that the Moon is casting a long shadow. Usually 00:01:58.780 --> 00:02:03.680 that shadow misses the Earth, but during an eclipse the Moon’s shadow falls on the Earth’s surface. 00:02:03.689 --> 00:02:08.950 In fact, there are two shadows from the Moon, one inside the other. One is a narrow cone, 00:02:08.950 --> 00:02:13.220 tapering to a point away from the Moon. If you’re anywhere physically inside this cone, 00:02:13.220 --> 00:02:16.880 the Moon appears big enough to completely block the Sun. That means this shadow is very 00:02:16.880 --> 00:02:21.520 dark, and we call it the umbra (which is Latin for – you guessed it – “shadow”). 00:02:21.520 --> 00:02:26.160 Outside of this deep umbral shadow is a wider conical region where, if you’re in it, the 00:02:26.160 --> 00:02:30.920 Sun is only partially blocked; you can still see some of the Sun past the Moon. You’re 00:02:30.930 --> 00:02:34.980 getting less light, and so you’re technically shadowed, but it’s not quite as dark as 00:02:34.980 --> 00:02:41.160 the umbra. This region is called the “penumbra”; “pen” in this case for Latin meaning “almost,” or “nearly.” 00:02:41.160 --> 00:02:46.020 When the umbra touches the Earth, we get a total solar eclipse. But what does that look like from the ground? 00:02:46.020 --> 00:02:50.540 You don’t get a total eclipse right away. First, the edge of the Moon slips in front 00:02:50.550 --> 00:02:55.260 of the Sun, and we see a little dip in the Sun’s limb, its edge as seen from Earth 00:02:55.260 --> 00:02:57.880 (that’s the start of the penumbra sweeping over you). 00:02:57.890 --> 00:03:03.530 As the Moon slowly moves, that dip grows, becoming a bite. The Sun becomes a thick crescent, 00:03:03.530 --> 00:03:04.590 then a thin one. 00:03:04.590 --> 00:03:09.500 As the Sun becomes an ever-thinner crescent, the sky begins to darken. Then, finally, the 00:03:09.500 --> 00:03:14.510 Moon’s black disk completely covers the Sun — the umbra sweeps over your location. 00:03:14.510 --> 00:03:17.170 And at that moment, totality begins. 00:03:17.170 --> 00:03:21.170 You might think that this just means the sky gets dark, and it’s like night outside for 00:03:21.170 --> 00:03:26.480 a while. But a total eclipse is far more than that. And that’s because of the Sun’s corona. 00:03:26.480 --> 00:03:30.940 As I’ll cover in more detail in a future episode, the corona is the sun’s atmosphere, 00:03:30.950 --> 00:03:37.060 an ethereally thin envelope of gas that stretches from the Sun’s surface into space for millions of kilometers. 00:03:37.060 --> 00:03:41.700 It’s really faint, and therefore usually completely overwhelmed by the intensely bright 00:03:41.709 --> 00:03:42.629 light from the Sun. 00:03:42.629 --> 00:03:47.319 But when the Moon blocks the Sun’s face, the corona becomes visible. It surrounds the 00:03:47.319 --> 00:03:52.110 Sun, filaments and tendrils extending into the sky, an incredibly beautiful sight. I 00:03:52.110 --> 00:03:56.409 know many people who have said it’s the most spectacular thing they have ever seen. 00:03:56.409 --> 00:04:01.009 And there’s more. The Moon’s edge isn’t smooth — there are craters and other depressions. 00:04:01.009 --> 00:04:06.440 Craters right at the Moon’s edge allow sunlight to stream past. We see these as bright patches 00:04:06.450 --> 00:04:10.250 around the eclipsed Sun, which are called Baily’s Beads - because they were first 00:04:10.250 --> 00:04:14.370 described by English astronomer Francis Baily in 1836! 00:04:14.370 --> 00:04:19.199 Because the Moon and Sun are very nearly the same apparent size, totality is brief. 00:04:19.199 --> 00:04:22.909 The longest it can last is only about seven or eight minutes. That’s how long it takes 00:04:22.909 --> 00:04:27.210 the umbra to move over one spot on the Earth. When totality ends, and the Moon starts to 00:04:27.210 --> 00:04:32.009 move off of the Sun’s face, for a moment just a single spot of the Sun is unblocked, 00:04:32.009 --> 00:04:35.719 glowing fiercely on one side of the Moon. Sometimes you can get a circle of light around 00:04:35.719 --> 00:04:40.270 the Moon’s surface, and together with the bright spot it looks like a celestial wedding 00:04:40.270 --> 00:04:43.010 ring. In fact, this is called the Diamond Ring effect. 00:04:43.010 --> 00:04:47.300 Then, inexorably, the Moon pulls away from the Sun, and the order of events is reversed. 00:04:47.300 --> 00:04:51.839 The umbra is gone, but you’re still in the penumbral shadow. The Sun shows a thin crescent, 00:04:51.839 --> 00:04:55.409 then a thick one, then a dip in its side… and then it’s all over. 00:04:55.409 --> 00:04:59.619 The umbral shadow of the Moon is pretty small where it hits the Earth, so a total eclipse 00:04:59.619 --> 00:05:03.909 is a local event. If you’re too far north and south, you don’t get a total eclipse, 00:05:03.909 --> 00:05:09.029 you only get a partial one. Which is still cool, but lacks the mystique of a total eclipse. 00:05:09.029 --> 00:05:13.400 Remember too that the Moon’s orbit around the Earth is an ellipse. That means sometimes 00:05:13.409 --> 00:05:15.639 it’s closer to the Earth, and sometimes farther. 00:05:15.639 --> 00:05:19.330 If a solar eclipse happens when the Moon is at the far end of its orbit, it can actually 00:05:19.330 --> 00:05:24.259 be smaller than the Sun in the sky. It doesn’t block the entire face of the Sun, and it leaves 00:05:24.259 --> 00:05:26.889 a ring of light around the black circle of the Moon. 00:05:26.889 --> 00:05:31.589 This technical name for this shape is annulus, so this event is called an annular eclipse. 00:05:31.589 --> 00:05:36.029 A lot of people think if you look at a total solar eclipse you can go permanently and completely 00:05:36.029 --> 00:05:42.149 blind. That’s really not true. But, some parts of eclipse-watching are more dangerous than others. 00:05:42.149 --> 00:05:46.419 I mean, obviously it’s not a good idea to stand there and stare at the sun. Looking 00:05:46.419 --> 00:05:50.990 at even the uneclipsed Sun for more than a moment is painful, and that pain is the result 00:05:50.990 --> 00:05:55.450 of the damage that solar radiation is doing to your retinas. So I don’t recommend it 00:05:55.450 --> 00:05:56.100 — Duh. 00:05:56.110 --> 00:06:01.249 But when viewing an eclipse, the real concern is right after totality ends. During totality 00:06:01.249 --> 00:06:05.360 it’s dark, so your pupils have dilated to let more light in. But then there’s the 00:06:05.360 --> 00:06:09.879 flash of sunlight when the Moon moves off, and that’s intense enough to damage your retinas. 00:06:09.879 --> 00:06:14.429 That’s why astronomers recommend extreme caution when viewing an eclipse; because that 00:06:14.429 --> 00:06:16.149 flash can catch you by surprise. 00:06:16.149 --> 00:06:20.580 When viewing the Sun, don’t just stand there and stare at it; you should always have eye 00:06:20.580 --> 00:06:25.360 protection. And make sure you have safety-approved filters; don’t try the the home-made tricks 00:06:25.360 --> 00:06:30.119 you might have heard of -- like looking through an old CD or DVD, or using old-style camera 00:06:30.119 --> 00:06:31.080 film as a filter. 00:06:31.080 --> 00:06:35.119 These can let through too much infrared and ultraviolet light, and again can dilate your 00:06:35.119 --> 00:06:37.369 pupils, actually making things worse. 00:06:37.369 --> 00:06:41.490 Lots of companies make inexpensive filters that are great for Sun-spotting; we have links 00:06:41.490 --> 00:06:43.779 in dooblydoo for more information on eye safety. 00:06:43.779 --> 00:06:48.139 Now, you don’t have to worry about hurting your eyes at all when viewing a lunar eclipse. 00:06:48.139 --> 00:06:52.429 Because, in that case, it’s the Earth that blocks the Sun, and the Earth’s shadow falls 00:06:52.429 --> 00:06:54.349 on the Moon. So go nuts. 00:06:54.349 --> 00:06:58.099 But one big difference between the two kinds of eclipses is who can see them. 00:06:58.099 --> 00:07:03.379 A solar eclipse is localized to one spot on the Earth, or really a swath along the ground 00:07:03.379 --> 00:07:06.469 as the Moon’s umbral shadow sweeps across the Earth’s surface. 00:07:06.469 --> 00:07:10.669 But a lunar eclipse is when the Moon moves into Earth’s shadow, so anyone on Earth 00:07:10.669 --> 00:07:15.439 facing the Moon can see a lunar eclipse. This is why I’ve seen dozens of lunar eclipses 00:07:15.439 --> 00:07:20.599 but never a total solar one. I’ve never been at the right place at the right time. 00:07:20.600 --> 00:07:21.580 Not that I’m bitter. 00:07:21.580 --> 00:07:25.900 The Earth has umbral and penumbral shadows, too. When the Moon first enters the Earth’s 00:07:25.909 --> 00:07:29.909 penumbra, the dimming is so slight you hardly notice it. But as the Moon moves deeper into 00:07:29.909 --> 00:07:34.349 the penumbra, it starts to darken. Sometimes it changes color, turning a deep orange or 00:07:34.349 --> 00:07:35.289 blood red. 00:07:35.289 --> 00:07:38.569 That’s because the Earth is starting to block the sunlight heading toward the moon, 00:07:38.569 --> 00:07:42.440 and the only light that gets through is coming through the thickest part of our atmosphere. 00:07:42.440 --> 00:07:46.020 This blocks blue and green light, leaving only red to come through. 00:07:46.020 --> 00:07:50.100 That’s why the Moon and Sun look red to us when they’re on the horizon, rising and 00:07:50.100 --> 00:07:55.099 setting, too. When you look upon the red eclipsed Moon, you’re seeing the light from all the 00:07:55.099 --> 00:07:59.999 sunrises and sunsets in the world hitting the Moon and reflecting back to us. 00:07:59.999 --> 00:08:04.339 Finally, the Moon starts to enter the Earth’s umbra, and the real eclipse begins. At first 00:08:04.339 --> 00:08:08.369 it looks like a bite is taken out of it — that curving arc is the shadow of the edge of the 00:08:08.369 --> 00:08:12.689 Earth! The Moon moves deeper and deeper into the shadow until it’s completely darkened. 00:08:12.689 --> 00:08:16.089 The Earth is bigger than the Moon, so the Earth’s umbra is much wider; while a solar 00:08:16.089 --> 00:08:20.639 eclipse is over in minutes, a total lunar eclipse can last nearly two hours. I once 00:08:20.639 --> 00:08:24.849 saw a lunar eclipse so deep that it took me a minute to find the Moon in the sky! 00:08:24.849 --> 00:08:28.719 There’s not a lot of new science you can do with a lunar eclipse. But if you know a 00:08:28.719 --> 00:08:33.219 little geometry, you can use the size and shape of the Earth’s shadow on the Moon 00:08:33.219 --> 00:08:35.539 to get the relative sizes of the Earth and Moon. 00:08:35.540 --> 00:08:40.480 Ancient Greeks did just this, and got a number that wasn’t too far off. They also knew 00:08:40.490 --> 00:08:44.399 how big the Earth was using other methods, and so they had a decent estimate for the 00:08:44.399 --> 00:08:48.980 size of the Moon…nearly 2000 years before the invention of the telescope! 00:08:48.980 --> 00:08:53.100 They also knew the shape of the Earth’s shadow was always a circle, which only makes 00:08:53.100 --> 00:08:57.680 sense if the Earth were a sphere. If the Earth were flat, it would sometimes cast a thin 00:08:57.680 --> 00:09:01.360 shadow, but it never does. Pretty clever, those ancient Greeks. 00:09:01.360 --> 00:09:04.930 One final note. Because of tides from the Earth — which we’ll learn more about in 00:09:04.930 --> 00:09:09.389 detail in a later episode — the Moon is slowly moving away from the Earth, by about 00:09:09.389 --> 00:09:11.449 4 centimeters a year. 00:09:11.449 --> 00:09:16.740 As it recedes, it’s slowly getting smaller in the sky. This means that, eventually, it 00:09:16.750 --> 00:09:21.750 will be too far away to completely cover the Sun, and we won’t get any more total eclipses. 00:09:21.750 --> 00:09:26.580 Doing the rough math, that will be in about a billion years. Better watch eclipses while you can. 00:09:26.580 --> 00:09:30.399 Today you learned that a solar eclipse is when the Moon blocks the Sun so its shadow 00:09:30.400 --> 00:09:34.780 falls on the Earth, and a lunar eclipse is when the Earth’s shadow falls on the Moon. 00:09:34.780 --> 00:09:38.380 We don’t get them every two weeks because the Moon’s orbit is tilted. And if you’re 00:09:38.390 --> 00:09:41.890 clever, you can use lunar eclipses to figure out how big the Earth and Moon are. 00:09:41.890 --> 00:09:47.031 This episode is brought to you by Squarespace. The latest version of their platform, Squarespace Seven, has 00:09:47.031 --> 00:09:52.491 a completely redesigned interface, integrations with Getty Images and Google Apps, new templates, and 00:09:52.500 --> 00:09:58.020 a new feature called Cover Pages. Try Squarespace at Squarespace.com, and enter the code Crash Course at 00:09:58.020 --> 00:10:02.060 checkout for a special offer. Squarespace. Start Here. Go Anywhere. 00:10:02.060 --> 00:10:07.020 Crash Course Astronomy is produced in association with PBS Digital Studios. Head on over to their channel 00:10:07.020 --> 00:10:12.460 and discover more awesome videos. This episode was written by me, Phil Plait. The script was edited by 00:10:12.460 --> 00:10:17.040 Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was co-directed by Nicholas Jenkins 00:10:17.040 --> 00:10:21.100 and Michael Aranda, edited by Nicole Sweeney, and the graphics team is Thought Café.