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