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The big question in the comments last week was, “BUT WHAT ABOUT ECLIPSES?” Today, Phil breaks ’em down for you.
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Transcript Provided by YouTube:
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We humans of planet Earth benefit from a great coincidence. It’s a coincidence of time,
00:07
and of space. And of math.
00:09
The coincidence is this: the Sun is about 400 times wider than the Moon, and it’s
00:14
also on average about 400 times farther away than the Moon.
00:18
The apparent size of an object in the sky depends on how big it is and how far away
00:22
it is… so these numbers being equal means the Sun and the Moon appear to be about the
00:26
same size in the sky.
00:28
And that’s where another interesting thing comes in: Sometimes, the Moon passes directly
00:32
between the Earth and the Sun. It doesn’t happen all that often, but when it does, you get magic.
00:38
Or even better: You get SCIENCE.
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You get an eclipse.
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An eclipse is a generic term in astronomy for when one object passes into the shadow
00:56
of another object, darkening or blocking it.
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A solar eclipse is when the Moon blocks the Sun, casting a shadow on the Earth, and a
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lunar eclipse is when the Earth blocks the Sun, casting a shadow on the Moon.
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But how do they work?
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Well, the Moon orbits the Earth once per month, and the Earth orbits the Sun once per year.
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If the Moon’s orbit were perfectly aligned with the Earth’s, essentially sharing the
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same plane, we’d get a solar eclipse every new Moon and a lunar eclipse every full Moon!
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But we don’t. That’s because the Moon’s orbit is tilted with respect to Earth’s,
01:26
by about 5°.
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What that means is that, at new Moon, the Moon can be as much as 5° away from the Sun,
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passing “above” or “below” the Sun in the sky, thereby missing it, from our perspective.
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But sometimes the Moon is in the right place at the right time, and at new Moon, it lies
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perfectly in line between the Sun and the Earth. And when that happens, we get a solar
01:48
eclipse. This geometry happens at least twice per year, and sometimes as much as five times per year.
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What’s happening physically in space is that the Moon is casting a long shadow. Usually
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that shadow misses the Earth, but during an eclipse the Moon’s shadow falls on the Earth’s surface.
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In fact, there are two shadows from the Moon, one inside the other. One is a narrow cone,
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tapering to a point away from the Moon. If you’re anywhere physically inside this cone,
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the Moon appears big enough to completely block the Sun. That means this shadow is very
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dark, and we call it the umbra (which is Latin for – you guessed it – “shadow”).
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Outside of this deep umbral shadow is a wider conical region where, if you’re in it, the
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Sun is only partially blocked; you can still see some of the Sun past the Moon. You’re
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getting less light, and so you’re technically shadowed, but it’s not quite as dark as
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the umbra. This region is called the “penumbra”; “pen” in this case for Latin meaning “almost,” or “nearly.”
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When the umbra touches the Earth, we get a total solar eclipse. But what does that look like from the ground?
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You don’t get a total eclipse right away. First, the edge of the Moon slips in front
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of the Sun, and we see a little dip in the Sun’s limb, its edge as seen from Earth
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(that’s the start of the penumbra sweeping over you).
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As the Moon slowly moves, that dip grows, becoming a bite. The Sun becomes a thick crescent,
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then a thin one.
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As the Sun becomes an ever-thinner crescent, the sky begins to darken. Then, finally, the
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Moon’s black disk completely covers the Sun — the umbra sweeps over your location.
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And at that moment, totality begins.
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You might think that this just means the sky gets dark, and it’s like night outside for
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a while. But a total eclipse is far more than that. And that’s because of the Sun’s corona.
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As I’ll cover in more detail in a future episode, the corona is the sun’s atmosphere,
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an ethereally thin envelope of gas that stretches from the Sun’s surface into space for millions of kilometers.
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It’s really faint, and therefore usually completely overwhelmed by the intensely bright
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light from the Sun.
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But when the Moon blocks the Sun’s face, the corona becomes visible. It surrounds the
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Sun, filaments and tendrils extending into the sky, an incredibly beautiful sight. I
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know many people who have said it’s the most spectacular thing they have ever seen.
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And there’s more. The Moon’s edge isn’t smooth — there are craters and other depressions.
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Craters right at the Moon’s edge allow sunlight to stream past. We see these as bright patches
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around the eclipsed Sun, which are called Baily’s Beads – because they were first
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described by English astronomer Francis Baily in 1836!
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Because the Moon and Sun are very nearly the same apparent size, totality is brief.
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The longest it can last is only about seven or eight minutes. That’s how long it takes
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the umbra to move over one spot on the Earth. When totality ends, and the Moon starts to
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move off of the Sun’s face, for a moment just a single spot of the Sun is unblocked,
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glowing fiercely on one side of the Moon. Sometimes you can get a circle of light around
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the Moon’s surface, and together with the bright spot it looks like a celestial wedding
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ring. In fact, this is called the Diamond Ring effect.
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Then, inexorably, the Moon pulls away from the Sun, and the order of events is reversed.
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The umbra is gone, but you’re still in the penumbral shadow. The Sun shows a thin crescent,
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then a thick one, then a dip in its side… and then it’s all over.
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The umbral shadow of the Moon is pretty small where it hits the Earth, so a total eclipse
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is a local event. If you’re too far north and south, you don’t get a total eclipse,
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you only get a partial one. Which is still cool, but lacks the mystique of a total eclipse.
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Remember too that the Moon’s orbit around the Earth is an ellipse. That means sometimes
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it’s closer to the Earth, and sometimes farther.
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If a solar eclipse happens when the Moon is at the far end of its orbit, it can actually
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be smaller than the Sun in the sky. It doesn’t block the entire face of the Sun, and it leaves
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a ring of light around the black circle of the Moon.
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This technical name for this shape is annulus, so this event is called an annular eclipse.
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A lot of people think if you look at a total solar eclipse you can go permanently and completely
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blind. That’s really not true. But, some parts of eclipse-watching are more dangerous than others.
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I mean, obviously it’s not a good idea to stand there and stare at the sun. Looking
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at even the uneclipsed Sun for more than a moment is painful, and that pain is the result
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of the damage that solar radiation is doing to your retinas. So I don’t recommend it
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— Duh.
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But when viewing an eclipse, the real concern is right after totality ends. During totality
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it’s dark, so your pupils have dilated to let more light in. But then there’s the
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flash of sunlight when the Moon moves off, and that’s intense enough to damage your retinas.
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That’s why astronomers recommend extreme caution when viewing an eclipse; because that
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flash can catch you by surprise.
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When viewing the Sun, don’t just stand there and stare at it; you should always have eye
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protection. And make sure you have safety-approved filters; don’t try the the home-made tricks
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you might have heard of — like looking through an old CD or DVD, or using old-style camera
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film as a filter.
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These can let through too much infrared and ultraviolet light, and again can dilate your
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pupils, actually making things worse.
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Lots of companies make inexpensive filters that are great for Sun-spotting; we have links
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in dooblydoo for more information on eye safety.
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Now, you don’t have to worry about hurting your eyes at all when viewing a lunar eclipse.
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Because, in that case, it’s the Earth that blocks the Sun, and the Earth’s shadow falls
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on the Moon. So go nuts.
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But one big difference between the two kinds of eclipses is who can see them.
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A solar eclipse is localized to one spot on the Earth, or really a swath along the ground
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as the Moon’s umbral shadow sweeps across the Earth’s surface.
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But a lunar eclipse is when the Moon moves into Earth’s shadow, so anyone on Earth
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facing the Moon can see a lunar eclipse. This is why I’ve seen dozens of lunar eclipses
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but never a total solar one. I’ve never been at the right place at the right time.
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Not that I’m bitter.
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The Earth has umbral and penumbral shadows, too. When the Moon first enters the Earth’s
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penumbra, the dimming is so slight you hardly notice it. But as the Moon moves deeper into
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the penumbra, it starts to darken. Sometimes it changes color, turning a deep orange or
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blood red.
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That’s because the Earth is starting to block the sunlight heading toward the moon,
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and the only light that gets through is coming through the thickest part of our atmosphere.
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This blocks blue and green light, leaving only red to come through.
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That’s why the Moon and Sun look red to us when they’re on the horizon, rising and
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setting, too. When you look upon the red eclipsed Moon, you’re seeing the light from all the
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sunrises and sunsets in the world hitting the Moon and reflecting back to us.
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Finally, the Moon starts to enter the Earth’s umbra, and the real eclipse begins. At first
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it looks like a bite is taken out of it — that curving arc is the shadow of the edge of the
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Earth! The Moon moves deeper and deeper into the shadow until it’s completely darkened.
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The Earth is bigger than the Moon, so the Earth’s umbra is much wider; while a solar
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eclipse is over in minutes, a total lunar eclipse can last nearly two hours. I once
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saw a lunar eclipse so deep that it took me a minute to find the Moon in the sky!
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There’s not a lot of new science you can do with a lunar eclipse. But if you know a
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little geometry, you can use the size and shape of the Earth’s shadow on the Moon
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to get the relative sizes of the Earth and Moon.
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Ancient Greeks did just this, and got a number that wasn’t too far off. They also knew
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how big the Earth was using other methods, and so they had a decent estimate for the
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size of the Moon…nearly 2000 years before the invention of the telescope!
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They also knew the shape of the Earth’s shadow was always a circle, which only makes
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sense if the Earth were a sphere. If the Earth were flat, it would sometimes cast a thin
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shadow, but it never does. Pretty clever, those ancient Greeks.
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One final note. Because of tides from the Earth — which we’ll learn more about in
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detail in a later episode — the Moon is slowly moving away from the Earth, by about
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4 centimeters a year.
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As it recedes, it’s slowly getting smaller in the sky. This means that, eventually, it
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will be too far away to completely cover the Sun, and we won’t get any more total eclipses.
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Doing the rough math, that will be in about a billion years. Better watch eclipses while you can.
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Today you learned that a solar eclipse is when the Moon blocks the Sun so its shadow
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falls on the Earth, and a lunar eclipse is when the Earth’s shadow falls on the Moon.
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We don’t get them every two weeks because the Moon’s orbit is tilted. And if you’re
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clever, you can use lunar eclipses to figure out how big the Earth and Moon are.
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This episode is brought to you by Squarespace. The latest version of their platform, Squarespace Seven, has
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Crash Course Astronomy is produced in association with PBS Digital Studios. Head on over to their channel
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and discover more awesome videos. This episode was written by me, Phil Plait. The script was edited by
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Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was co-directed by Nicholas Jenkins
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and Michael Aranda, edited by Nicole Sweeney, and the graphics team is Thought Café.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.
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