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Eclipses – The Cosmic Coincidence · Talking Science

0:00
So I want you to picture yourself on a just a
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really warm, lazy afternoon. You're sitting right
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under this large leafy tree. Oh, nice. Setting
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the scene. Right. And if you look down at the
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footpath around you, you'll see what you always
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see under a tree on a sunny day. Just thousands
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of those ordinary scattered round patches of
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dappled light filtering down through the canopy.
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Yeah, totally mundane. everyday physics exactly
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it's a completely mundane site but now picture
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that exact same afternoon that same tree but
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uh during a partial solar eclipse oh things get
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weird they get so weird the moon has just taken
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a little bite out of the sun's disc up in the
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sky right and if you look down at the footpath
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now suddenly All those patches of light are not
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round anymore. Not at all. Every single gap between
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the leaves above you has completely transformed.
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You are looking at a footpath scattered with
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thousands of tiny repeated crescents. It's honestly
0:55
one of the most beautiful accidental science
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demonstrations in all of nature. Right, because
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it is a perfect miniature copy of whatever shape
1:03
the sun currently is, just repeated over and
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over again on the ground. Yeah, and nobody arranged
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those leaves to do that. You know, it happens
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purely because of a phenomenon known as the pinhole
1:13
camera effect. OK, so break that down for us.
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For anyone who hasn't encountered that term before,
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what is a pinhole camera? Well, it's a very simple
1:20
optical trick, really. When light passes through
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a remarkably small opening. like the tiny gaps
1:27
between overlapping leaves, that opening actually
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acts like a simple optical lens. Wow. Yeah. It
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takes the light from the source, which in this
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case is the sun, and faithfully projects an inverted
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image of that source onto whatever surface happens
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to be behind it. So the ground. So instead of
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just letting a generic, you know. blob of light
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through, the leaves are literally acting as thousands
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of tiny natural movie projectors. That's a great
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way to put it. Yeah. They're beaming the live
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status of the sun directly onto the dirt. Which
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is really the perfect entry point for what we
2:00
are exploring today, because an eclipse itself
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is, I mean, it's really just a much grander version
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of that exact same basic idea. It really is.
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And the goal for this deep dive is to take an
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unhurried walk through the mathematically precise
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shadow play of eclipses. I love that. Shadow
2:17
play. Yeah. We're going to explore the geometry
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that makes them possible, the history of how
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early humans figured out the patterns, and just
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the sheer visceral mechanics. of these cosmic
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alignments. Well, to really understand that grand
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shadow play, we have to zoom out from that tree
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for a second and look at the solar system as
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a whole. Okay, zooming out. An eclipse, fundamentally,
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is just one object in space blocking light on
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its way to another object. It's casting a shadow
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shaped by very simple geometry. Right, and there
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are two main types, solar and lunar. And the
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difference basically comes down to which object
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is doing the blocking. Exactly. So in a solar
2:53
eclipse, the moon steps between the sun and earth.
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The moon's shadow falls on us. But in a lunar
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eclipse, Earth steps between the sun and the
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moon, and our massive shadow falls onto the lunar
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surface. Right. But if we start with solar eclipses,
3:07
there is this fundamental question of scale that
3:10
always kind of blows my mind. But the perfect
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fit. Yes. When you look up during a total solar
3:15
eclipse, the moon seems to perfectly, exactly
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cover the sun. It's not too big, not too small.
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Does the math just happen to work out that they
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are physically the same size? Oh, not even close.
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The sun is massive. I mean, it is roughly 400
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times wider than the moon. 400 times. Yeah, but
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there's this staggering geometric coincidence
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at play here. The sun is also, on average, roughly
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400 times further away from Earth than the moon
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is. Wait, let me make sure I'm visualizing this
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correctly. So it would be like holding a tiny
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little coin right at arm's length and realizing
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that this tiny coin perfectly covers up a massive
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dinner plate that somebody's holding all the
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way on the other side of a long room. That is
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precisely how it works. When you multiply those
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two facts together, the 400 times width and the
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400 times distance, you get this extraordinary
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visual trick. It's like an optical illusion built
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into the solar system. Exactly. Seen from our
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vantage point here on Earth, The sun and the
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moon appear to be almost exactly the same size
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in the sky. They are so perfectly matched that
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during a total solar eclipse, the moon can precisely
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cover the sun's bright disk. No more. And no
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less. Which, I mean, that raises the question
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of why. Is there some gravitational law or, I
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don't know, physics rule that mandates this exact
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ratio in planetary systems? Yeah, and that's
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the wild part. As far as anyone in astronomy
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or astrophysics can tell, there is absolutely
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no known physical reason why those two objects
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have to line up this neatly. Wait, really? It's
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just random. It is simply a pure coincidence
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of how our particular solar system happened to
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form. That is wild. Well, this perfect match
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isn't even a permanent fixture. Because the moon
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isn't stationary, right? Like it's slowly drifting.
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Exactly. The moon is gradually moving away from
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Earth over these extremely long time scales,
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which means that total eclipses, as we experience
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them right now, are a temporary feature. Wow.
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Yeah. In the very long run of our planet's multi
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-billion year history, this... Perfect alignment
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is just a passing phase. We just happen to exist
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during the exact right window of time to see
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it. Okay, so we have this perfect temporary alignment.
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But the moon is orbiting the Earth, completing
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a lap roughly once a month, right? Correct. And
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it passes between us and the sun during every
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single new moon phase. Right, exactly. So if
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it's passing between us and the sun every single
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month, why aren't we plunged into darkness every
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30 days? Right, that's the obvious question.
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And it all comes down to what you could call
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orbital fussiness. Orbital fussiness, I like
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that. The key is that the moon's orbit is not
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perfectly flat relative to Earth's orbit. Okay.
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If you imagine the flat plane that the Earth
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travels on as it orbits the sun. Astronomers
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call this flat plane the ecliptic. Ecliptic,
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right. Yeah. So the moon's orbit around Earth
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doesn't sit perfectly level on that ecliptic.
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It is actually tilted by about five degrees.
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Okay, so if I picture like two hula hoops, one
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inside the other. Like an analogy. The outer
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hula hoop is the Earth going around the sun.
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And the inner hula hoop is the moon going around
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the Earth. But the inner one is just slightly
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tipped. Yes. So most of the time, the edges just
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don't actually line up. Exactly. I mean, five
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degrees sounds pretty negligible, right? But
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in the vastness of space, it is huge. Right,
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because of the distance. Exactly. It's more than
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enough that during most months, when the new
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moon passes in front of the sun, from our perspective,
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it actually passes slightly above the sun. you
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know slightly below it so it just misses the
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mark completely yeah his shadow gets cast out
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into empty space completely missing earth what
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actually has to happen for the shadow to hit
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us then well for an eclipse to occur a new moon
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has to happen very close to what astronomers
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call nodes yeah nodes these are the exact specific
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intersection points where that tilted lunar orbit
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physically crosses the flat plane of the Earth's
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orbit. So you need the moon to be at the new
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moon phase, and you need it to be sitting right
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at one of those precise intersection nodes. That's
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why eclipses are these rare, geometrically fussy
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events, and not just a monthly routine. Right.
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But even when they do hit those nodes and cast
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a shadow on Earth, the shadow doesn't always
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look the same, right? Like, we don't always get
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a total eclipse. Right, exactly. Sometimes we
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get an annular eclipse or a partial one. But
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if the ratio of the sun and moon size is so perfect,
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like we just talked about, what causes those
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variations? That comes down to another slight
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imperfection in the system. The moon's orbit
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isn't a perfect circle. It is slightly elliptical,
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meaning it's a bit oval -shaped. Oh, I see. So
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the physical distance between the Earth and the
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moon actually changes a bit throughout the month.
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When the moon happens to be at its closest point
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to Earth during one of those node alignments,
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it appears very slightly larger in our sky. And
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that's what gives us a total solar eclipse. Yes.
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The moon completely covers the sun's disk, plunging
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the area beneath the shadow into that eerie,
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temporary daytime darkness. But if the moon happens
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to be on the further end of that oval orbit when
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it crosses the node, then it appears very slightly
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smaller in the sky. So it isn't quite big enough
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to cover the whole solar disk. Right. And this
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is what creates an annular eclipse. People often
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call it a ring of fire. Oh, yeah, the ring of
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fire. Those pictures are incredible. They really
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are. Because a brilliant, thin ring of actual
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sunlight remains visible around the dark silhouette
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of the moon the entire time. And a partial eclipse
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would just be when the alignment isn't perfectly
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dead center? Exactly. Or maybe the observer on
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Earth is standing off to the side, you know,
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in the outer weaker part of the moon shadow.
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The moon only covers a chunk of the sun. Which
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brings us right back to our opening under the
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tree, producing all those little crescent shapes
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on the footpath. Full circle. Yeah, it's all
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dictated by distance and angles. And because
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this geometry operates basically like clockwork,
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it is incredibly predictable once you actually
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know the mechanics. Right. But early humans didn't
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know the mechanics. No, they definitely did not.
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I mean, before we understood elliptical orbits
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and five degree tilts, an eclipse must have been
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deeply, deeply unsettling. Just imagine you're
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going about your day and the sun just gets eaten
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by a black void. It would be terrifying. There's
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actually an old Chinese legend about two court
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astronomers, Hai and Hou, and they were supposedly
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executed for failing to predict an eclipse and
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warn the emperor in time. Oh, wow. High stakes.
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Right. And whether that's folklore or hard history,
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it really illustrates how early societies viewed
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these events. They weren't fun science demonstrations.
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They were terrifying omens that demanded an urgent
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official response. And because ancient astronomers
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were so heavily motivated to see these omens
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coming, They started noticing patterns. Because
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their lives literally depended on it. Exactly.
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Even without knowing the underlying physics,
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they realized eclipses weren't entirely random.
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And this led to the discovery of the Saros Cycle.
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Okay, the Saros Cycle. How does that cycle actually
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work? It is a marvel of early human observation.
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It is a period of very close to 18 years, 11
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days, and 8 hours. 18 years, 11 days, and 8 hours.
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That is very specific. Extremely specific. And
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after that exact amount of time, the sun, the
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earth, and the moon return to a nearly identical
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relative geometric arrangement. Because of this,
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a very similar eclipse will recur. But the extra
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8 hours is the catch, though, right? Because
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8 hours is a third of a day. meaning the Earth
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has rotated a third of the way around its axis
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since the last eclipse. And you nailed it. So
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the shadow will fall on a completely different
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part of the globe. Exactly right. And the reason
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the Saro cycle works at all is because it represents
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the precise moment when three completely separate
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lunar rhythms happen to sync up simultaneously.
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Wow, okay. Yeah. You have the time it takes the
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moon to return to the same phase. You have the
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time it takes the moon to return to the same
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node, you know, that intersection point in its
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tilted orbit. And you have the time it takes
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the moon to return to the same distance from
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Earth in its oval -shaped orbit. So it's like
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having three different ticking clocks, all with
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slightly different lengths, and just waiting
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for the exact moment they all chime at the same
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second. That is a perfect analogy. What stands
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out to me is that Babylonian, Chinese, and Greek
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observers figured this out through... Through
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sheer brute force pattern spotting. It's incredible.
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They kept meticulous records over centuries just
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to predict these events, long before they understood
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gravity or orbits. Though, you know, we should
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give credit where it's due. The Greek philosopher
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Anaxagoras actually did correctly guess the geometry
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way back in the 5th century BCE. Wait, really?
11:53
The 5th century BCE? Yeah. He argued that a solar
11:57
eclipse was just the moon passing in front of
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the sun. It was a strikingly modern explanation
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for an ancient world that still largely relied
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on mythology. That is amazing. But if Anaxagoras
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was the start of understanding eclipses, the
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absolute pinnacle of eclipse science has to be
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1919. Oh, absolutely. Because this is the moment
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a shadow literally changed how we understand
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the fabric of the universe. You're referring
12:21
to Arthur Eddington's expedition. Yes. In May
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of 1919, the British astronomer Arthur Eddington
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led teams to the island of Principe off West
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Africa and to Sobral in Brazil, specifically
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to observe a total solar eclipse. But he wasn't
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there to study the eclipse itself, though, right?
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No, not at all. He was using the eclipse as a
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tool to test a brand new, highly controversial
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theory published by Albert Einstein. General
12:45
relativity. Precisely. Einstein's theory proposed
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something totally radical. that massy objects
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like our sun actually bend the physical fabric
12:55
of space and time around them. Which sounds completely
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wild. It sounded absurd to a lot of people. But
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because of that bent space, Einstein predicted
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that light traveling from distant stars should
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be deflected or bent very slightly as it passed
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close to the heavy mass of the sun on its way
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to our eyes here on Earth. Right, but the problem
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with testing that... on a normal tuesday is that
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you can't just look at stars right next to the
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sun no definitely not the sun's glare is blinding
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the sky is bright blue and the stars are completely
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invisible which is why totality is so crucial
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for this during those few minutes of a total
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solar eclipse the sun's bright disk is completely
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blocked by the moon the sky goes dark suddenly
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stars close to the sun's position in the sky
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become briefly visible right in the middle of
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the day. It's like turning off the lights in
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a room so you can see out the window. Exactly.
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So Eddington's team photographed those stars
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during the eclipse. Then they compared those
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photos against photos of the exact same stars
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taken months earlier at night when the sun was
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nowhere near them. And when they developed those
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photos, the stars in the eclipse image appeared
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to have shifted. Yes. They had moved by almost
14:06
the exact amount that Einstein's math predicted.
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The sheer mass of the sun had bent the light
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passing by it. It's just phenomenal. A shadow
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lasting just a few minutes provided the perfect
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natural laboratory to prove that space -time
14:19
is flexible. I mean... It established relativity
14:21
as serious, tested physics. It is a beautiful
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convergence of events. Yeah. It really highlights
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just how uniquely valuable totality is for science.
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And beyond the science, the experience of totality
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itself is described as just a genuinely startling,
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bizarre sensory event. Oh, it is. It's profoundly
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weird. It's not just that the sky gets a bit
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dark, right? In the minutes leading up to totality,
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the quality of the light fundamentally changes.
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It flattens out. Yes, it flattens. Shadows sharpen
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in a way that feels incredibly unnatural. You
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can actually feel the temperature measurably
14:55
drop as the sunlight is suddenly cut off. And
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then for anywhere from a few seconds to maybe
15:01
seven minutes, totality sets in. And to understand
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what you're seeing in that specific moment, we
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need to define two distinct parts of the sun.
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Okay. First, there's the photosphere. That is
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the sun's bright, normally visible disk. It's
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the blinding surface we're used to seeing. Right,
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the part that hurts to look at. Exactly. During
15:19
totality, that photosphere is completely blocked.
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And because it's blocked, something normally
15:24
invisible is unveiled. The corona. The corona
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being the sun's atmosphere. Right. It is this
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wispy pale halo of extremely hot, extremely thin
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plasma that extends millions of kilometers out
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into space from the sun's surface. And it's always
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there, isn't it? It is always there. Under normal
15:41
daylight conditions, it's happening right now.
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But it is roughly a million times dimmer than
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the photosphere. It is just... utterly overwhelmed
15:49
by the glare so totality removes the glare and
15:53
the corona appears exactly and for a huge chunk
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of the 20th century a total eclipse was basically
15:59
the only practical way for scientists to study
16:02
the corona at all it's that dramatic reveal that
16:04
drives you know eclipse chasers to travel the
16:06
globe heading to isolated places like xmith in
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western australia in 2023 or like preparing for
16:12
sydney in july 2028 oh yeah they plan years in
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advance they are chasing these fleeting visual
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phenomena that only happen for seconds, like
16:20
Bailey's beads. Oh, Bailey's beads are spectacular.
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They occur just before and just after totality.
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What causes them? Well, the moon doesn't have
16:28
a perfectly smooth edge, right? It is covered
16:30
in mountains and deep valleys. Oh, right. So
16:33
right at the edge of alignment, sunlight streams
16:36
through those deep lunar valleys, breaking up
16:39
into a scatter of brilliant, distinct points
16:41
of light around the moon's silhouette. It looks
16:44
like a string of glowing lamps. And then as the
16:47
very last bead fades right before totality, or
16:51
as the very first one reappears after, you get
16:54
what is called the diamond ring effect. Yes,
16:56
the classic diamond ring. It is one single intense
16:59
glowing point of light attached to this pale
17:02
ring. It is arguably the most photographed moment
17:05
of any eclipse. Oh, without a doubt. And while
17:08
you're watching this, birds are suddenly falling
17:09
silent, insects start chirping thinking it's
17:12
night, and you might even see shadow bands. Shadow
17:14
bands are such a strange phenomenon. They are
17:17
these fast -moving, rippling patterns of light
17:19
and shadow that can race across pale, flat surfaces
17:22
right around totality. Like ripples at the bottom
17:25
of a pool. Yes. Exactly like that. They're caused
17:29
by that tiny remaining sliver of sunlight being
17:32
refracted unevenly by turbulence in Earth's own
17:35
atmosphere. That is so cool. Okay, so if the
17:38
moon's shadow causes all this daytime drama,
17:40
what happens when we reverse the geometry? Let's
17:43
talk about lunar eclipses. The lunar reverse.
17:46
Right. Earth steps into the middle, and it's
17:49
our massive shadow blocking the sunlight from
17:51
hitting the full moon. The immediate question
17:53
is... If the Earth is completely blocking the
17:56
sun, why doesn't the moon just go pitch black?
17:59
That's a great question. Why does it turn that
18:01
famous deep coppery red, you know, the blood
18:04
moon? The answer to that lies in our own planet's
18:07
atmosphere through a process called Rayleigh
18:09
scattering. Okay, walk us through how Rayleigh
18:12
scattering turns a dark shadow red. Sure. So
18:15
as sunlight travels toward Earth, it hits our
18:17
atmosphere. And our atmosphere acts like a lens
18:19
and a filter combined. It bends some of that
18:22
light inward. toward the shadow behind Earth.
18:25
But as it does this, it filters out the shorter,
18:28
bluer wavelengths of light. This exact scattering
18:31
of blue light is actually the reason our daytime
18:33
sky looks blue to us down on the ground. Because
18:36
the blue light gets trapped and scattered around
18:38
in the atmosphere. Exactly. So if the atmosphere
18:41
scatters away all the blue light, what's left
18:44
to pass through? The longer wavelengths. The
18:46
reds and oranges. Yes. The reds, the oranges.
18:50
The coppery colors. This reddish light survives
18:53
the trip through the edges of Earth's atmosphere,
18:55
gets bent into our shadow by refraction, and
18:58
falls squarely onto the moon's surface. So the
19:00
Earth's atmosphere is basically acting like a
19:02
giant cosmic color filter. Pretty much. It tosses
19:05
out the blue light and it bends the reddish light
19:07
from the edges of the Earth. It's projecting
19:09
the collective light of all the world's sunsets
19:11
and sunrises onto the moon at the exact same
19:14
time. That is an incredibly accurate way to describe
19:16
it. A blood moon is quite literally illuminated
19:19
by every sunset and sunrise happening on Earth
19:22
in that specific moment. That is almost poetic.
19:25
It really is. And what makes lunar eclipses so
19:27
accessible is their scale. Earth's shadow is
19:30
massive compared to the moon. Right. So when
19:34
a lunar eclipse happens, you don't have to travel
19:36
to a tiny specific path like you do for a total
19:39
solar eclipse. A lunar eclipse is visible to
19:42
the entire nighttime hemisphere of Earth simultaneously.
19:45
And it lasts for hours, not minutes. You literally
19:48
just walk outside and look up, which actually
19:51
brings us to a really critical safety point here.
19:53
Yes, very important. Looking at a lunar eclipse
19:55
is perfectly safe. Looking at a solar eclipse,
19:58
however, requires understanding a very real biological
20:01
danger. We have to be very direct about this.
20:04
Looking directly at the sun at any point during
20:07
a partial solar eclipse or even during the partial
20:09
phases leading up to and following totality is
20:12
intensely dangerous to your eyesight. Even if
20:15
the sun looks dimmer or a large chunk of it is
20:17
missing? the invisible radiation is still hitting
20:20
your eye. And the scariest part is that you won't
20:23
feel it happening. No, you won't. The retina,
20:25
which is the light -sensitive tissue at the back
20:27
of your eye, has absolutely no pain receptors.
20:29
None. So you just don't feel it burning. Exactly.
20:32
This means you can literally burn permanent blind
20:34
spots into your vision without feeling a single
20:37
thing while the damage is occurring. The damage
20:40
is silent and it is permanent. And regular sunglasses
20:42
don't help, right? Ordinary sunglasses, regardless
20:45
of how dark or how expensive they are, offer
20:48
zero protection against this specific kind of
20:52
solar radiation. So to view a partial solar eclipse
20:55
safely... You must use purpose -built certified
20:59
solar filters, you know, the specific cardboard
21:02
eclipse glasses with international safety ratings.
21:04
Yes. Or you go back to where we started this
21:06
deep dive, indirect viewing. You make a pinhole
21:09
projector with a piece of card and you safely
21:11
watch the projection on the ground, just like
21:14
the leaves do. There is one and only one genuine
21:18
exception to this rule. And that's totality.
21:20
Yes. The brief window of 100 % totality. Only
21:25
when the sun's bright disk is completely, totally
21:28
covered by the moon does it become safe and truly
21:31
spectacular to look directly at the Carina with
21:33
the naked eye. But you have to be absolutely
21:35
certain totality has begun. Exactly. And the
21:39
second that diamond ring flashes and the sun
21:41
starts to reappear, the protective filters must
21:44
go straight back on. It requires care, but the
21:47
payoff of seeing the corona is apparently just
21:49
life -changing. It's unparalleled. So, as we
21:53
wrap up our journey through the shadows today,
21:55
I want to bring you back to that warm afternoon.
21:57
Put yourself back under that leafy tree. Right
22:00
back where we started. The next time you find
22:02
yourself standing under a canopy on an ordinary
22:05
sunny afternoon, remember what is actually happening.
22:09
Every single gap in the leaves above you is quietly
22:12
acting as a camera, faithfully tracing the truth
22:15
of the cosmos right onto the ground at your feet,
22:17
whether anything unusual is happening up there
22:19
or not. It is a quiet, ongoing record of our
22:23
solar system's mechanics. But, you know, understanding
22:26
these incredibly precise mechanics opens up one
22:29
final, fascinating reality to think about. Oh,
22:32
what's that? We talked earlier about how the
22:34
moon is gradually moving away from the Earth,
22:36
right? Right, the slow drift. Because of that
22:38
drift, We know that one day, millions of years
22:41
from now, the moon will be too far away to ever
22:44
fully cover the sun's disk. Total eclipses will
22:47
simply cease to exist on Earth. Wow. Future beings
22:50
will only ever see partial or annular eclipses,
22:53
never a total one. That's sort of heartbreaking,
22:56
but also amazing. It really is. And it invites
22:58
a pretty profound question. What other temporary
23:01
cosmic masterpieces are we just taking for granted
23:04
right now, simply because our tiny slice of human
23:07
history happened to align perfectly with the
23:10
clockwork of the universe? Something to mull
23:12
over the next time you're standing in the shade.