Eclipses: Crash Course Astronomy #5

Summary

This script explores the fascinating phenomenon of eclipses, highlighting the rare alignment of the Sun and Moon's apparent sizes due to their relative distances. It explains the mechanics of solar and lunar eclipses, the Moon's tilted orbit, and the different types of shadows cast, including the dramatic total solar eclipse and its beautiful corona. The script also discusses safety during eclipse viewing, the global visibility of lunar eclipses, and the historical use of eclipses to determine the Earth and Moon's sizes, ending with a note on the Moon's gradual drift away from Earth.

Takeaways

• 🌞 The Sun and the Moon appear to be about the same size in the sky due to the Sun being approximately 400 times wider than the Moon and also about 400 times farther away.
• 🌕 An eclipse occurs when the Moon passes directly between the Earth and the Sun, resulting in a solar eclipse, or when the Earth blocks the Sun's light from reaching the Moon, resulting in a lunar eclipse.
• 🛰️ The Moon's orbit is tilted about 5° with respect to Earth's, which is why solar and lunar eclipses do not occur every new and full Moon respectively.
• 🌑 During a solar eclipse, the Moon casts an umbra, a dark shadow, and a penumbra, a lighter shadow, on Earth, with the umbra allowing for a total solar eclipse where the Sun is completely blocked.
• 🌓 The experience of a total solar eclipse includes the visibility of the Sun's corona, an outer atmosphere, and phenomena like Baily's Beads as the Moon's edge covers the Sun.
• 💫 The 'Diamond Ring' effect is observed during a solar eclipse when a small portion of the Sun is visible as a bright spot just as the Moon moves off the Sun's face.
• 👀 Viewing a solar eclipse requires extreme caution and proper eye protection to prevent damage to the retinas from intense sunlight, especially at the end of totality.
• 🌌 A lunar eclipse is visible to anyone on Earth facing the Moon, as it involves the Moon moving into Earth's shadow, which can give the Moon a deep orange or red hue due to the scattering of light by the Earth's atmosphere.
• 🌍 The Earth's shadow on the Moon during a lunar eclipse can be used to estimate the relative sizes of the Earth and Moon, a method used by ancient Greeks to understand the cosmos.
• 🌑 The Moon is slowly moving away from the Earth at a rate of about 4 centimeters per year, which will eventually prevent total solar eclipses from occurring in about a billion years.
• 🌐 The script is sponsored by Squarespace and is part of the Crash Course Astronomy series, highlighting the educational and entertaining nature of the content.

Q & A

Why do solar and lunar eclipses occur?

Solar and lunar eclipses occur due to the alignment of the Earth, Moon, and Sun. A solar eclipse happens when the Moon passes between the Earth and the Sun, casting a shadow on the Earth. A lunar eclipse occurs when the Earth is between the Moon and the Sun, casting a shadow on the Moon.

What is the significance of the Sun and Moon appearing to be the same size in the sky?

The Sun and Moon appearing to be the same size in the sky is significant because it allows for total solar eclipses. The Sun is about 400 times wider than the Moon and also about 400 times farther away, which makes their apparent sizes equal, enabling the Moon to completely cover the Sun during an eclipse.

Why don't we have solar and lunar eclipses every new and full moon respectively?

We don't have eclipses every new and full moon because the Moon's orbit is tilted about 5° relative to Earth's orbit. This tilt means the Moon can pass above or below the Sun in the sky, missing alignment and thus not casting a shadow on the Earth or Moon.

What are the two types of shadows cast by the Moon during an eclipse?

The two types of shadows cast by the Moon are the umbra and the penumbra. The umbra is a narrow, dark shadow cone where the Moon completely blocks the Sun, while the penumbra is a wider, lighter shadow cone where the Sun is only partially blocked.

What is the phenomenon known as 'totality' during a solar eclipse?

Totality is the brief period during a solar eclipse when the Moon completely covers the Sun, allowing the Sun's faint corona to become visible. This is considered the most spectacular part of a total solar eclipse.

What are Baily's Beads, and how do they occur during a solar eclipse?

Baily's Beads are bright patches of sunlight that appear around the eclipsed Sun just before and after totality. They occur due to the Moon's uneven edge, particularly its craters, which allow sunlight to stream past and create these bright spots.

What is the Diamond Ring effect, and when does it occur during a solar eclipse?

The Diamond Ring effect occurs just as totality ends during a solar eclipse. It is when a single point of the Sun is unblocked, creating a bright spot on one side of the Moon, which, together with the surrounding light, resembles a diamond ring.

Why is it dangerous to look at the Sun without proper eye protection during an eclipse?

Looking at the Sun without proper eye protection can be dangerous because the intense solar radiation can damage the retinas. This is especially risky during an eclipse because after totality, when the Moon moves off, the sudden flash of sunlight can catch viewers by surprise and cause harm.

How can the Earth's shadow during a lunar eclipse be used to determine the relative sizes of the Earth and Moon?

During a lunar eclipse, the size and shape of the Earth's shadow on the Moon can be used to determine the relative sizes of the Earth and Moon through geometric calculations. Ancient Greeks used this method to estimate the sizes of these celestial bodies.

Why do the Moon and Sun appear red when they are on the horizon?

The Moon and Sun appear red when they are on the horizon because of the scattering of light by the Earth's atmosphere. Blue and green light are scattered away, leaving only the red light to reach the observer, which gives the celestial bodies a reddish hue.

What is the long-term effect of the Moon moving away from the Earth on total solar eclipses?

As the Moon slowly moves away from the Earth, it will eventually be too far away to completely cover the Sun, meaning that there will come a time, estimated to be about a billion years from now, when total solar eclipses will no longer occur.

Outlines

00:00

🌞 The Science of Solar and Lunar Eclipses

This paragraph explains the rare alignment of the Sun and Moon, which appear the same size in the sky due to the Sun being 400 times larger but also 400 times farther from Earth. It delves into the mechanics of solar and lunar eclipses, the tilt of the Moon's orbit, and the resulting shadows cast on Earth. The inner shadow, or umbra, creates a total solar eclipse, while the outer shadow, or penumbra, causes a partial eclipse. The paragraph also describes the visual spectacle of a total solar eclipse, including the visibility of the Sun's corona and Baily's Beads, and the brief duration of totality. It concludes with the local nature of total solar eclipses and the phenomenon of the 'Diamond Ring' effect.

05:04

🌑 The Dynamics of Eclipses and Viewing Safety

The second paragraph discusses the elliptical nature of the Moon's orbit and the resulting annular eclipse when the Moon is too small to cover the Sun. It addresses common misconceptions about the dangers of viewing an eclipse, emphasizing the importance of eye protection during solar eclipses to prevent retinal damage. The paragraph also contrasts solar and lunar eclipses in terms of visibility, with the latter being visible to anyone on Earth facing the Moon. It describes the gradual dimming and color change during a lunar eclipse, attributed to the Earth's atmosphere filtering sunlight. The paragraph concludes with historical insights into the Earth and Moon's relative sizes and shapes, deduced by ancient Greeks using lunar eclipses, and notes the Moon's gradual drift away from Earth, which will eventually end total solar eclipses in about a billion years.

10:07

🎬 Behind the Scenes of Crash Course Astronomy

The final paragraph provides credits for the production of Crash Course Astronomy, acknowledging the contributions of various individuals and teams. It mentions the script editor Blake de Pastino, consultant Dr. Michelle Thaller, co-directors Nicholas Jenkins and Michael Aranda, editor Nicole Sweeney, and the graphics team from Thought Café. The paragraph also promotes Squarespace, highlighting its platform's features and offering a special discount code for viewers.

Keywords

💡Eclipse

An eclipse is a celestial event where one object moves into the shadow of another, significantly darkening or obscuring it. In the context of the video, it explains the occurrence of both solar and lunar eclipses, which are central to the video's theme of celestial mechanics and astronomical phenomena.

💡Solar Eclipse

A solar eclipse happens when the Moon passes between the Earth and the Sun, casting a shadow on the Earth and blocking the Sun's light. It is a key concept in the video, illustrating the alignment of celestial bodies and the resulting visual effects observed from Earth.

💡Lunar Eclipse

A lunar eclipse occurs when the Earth is positioned between the Sun and the Moon, casting its shadow on the Moon. The video uses this concept to contrast with a solar eclipse and to explain the different viewing experiences and scientific observations possible during each type of eclipse.

💡Umbra

The umbra is the dark, central part of a shadow where the light source is completely blocked by an object. In the video, it is used to describe the shadow cast by the Moon during a solar eclipse, where the Sun is entirely obscured, leading to the unique astronomical event of totality.

💡Penumbra

The penumbra is the outer, lighter part of a shadow where only a portion of the light source is blocked. The video explains that during an eclipse, the penumbra causes a partial obscuring of the Sun, which is visible to observers outside the path of totality.

💡Totality

Totality refers to the brief period during a total solar eclipse when the Moon completely covers the Sun's disk. The video emphasizes the significance of this moment, highlighting the visibility of the Sun's corona and the dramatic change in the environment.

💡Corona

The corona is the outer atmosphere of the Sun, visible only when the Sun's photosphere is obscured during a total solar eclipse. The video describes the corona's appearance as a beautiful sight of filaments extending from the Sun, a key aspect of the totality experience.

💡Baily's Beads

Baily's Beads are bright spots of light seen during a total solar eclipse just before totality, caused by sunlight shining through lunar valleys. The video uses this phenomenon to illustrate the irregularities of the Moon's edge and the dramatic visual effect it creates.

💡Annular Eclipse

An annular eclipse occurs when the Moon is too far from the Earth to completely cover the Sun, leaving a ring of sunlight visible around the Moon. The video explains this as a type of solar eclipse that results from the Moon's elliptical orbit and its varying distance from Earth.

💡Eye Safety

The video emphasizes the importance of eye safety when viewing an eclipse, particularly the risk of retinal damage from staring at the Sun. It advises the use of safety-approved filters and warns against homemade methods, underscoring the need for proper protection during astronomical observations.

💡Tilted Orbit

The Moon's orbit is tilted about 5° relative to the Earth's, which is why solar and lunar eclipses do not occur every new and full moon. The video uses this concept to explain the rarity and alignment requirements for these celestial events, tying it to the broader theme of Earth's place in the cosmos.

Highlights

The Sun and the Moon appear to be about the same size in the sky due to the Sun being 400 times wider and 400 times farther away than the Moon.

An eclipse occurs when the Moon passes directly between the Earth and the Sun, creating a solar eclipse, or the Earth blocks the Sun casting a shadow on the Moon, creating a lunar eclipse.

The Moon's orbit is tilted by about 5° relative to Earth's, preventing solar and lunar eclipses from happening every new and full Moon.

A solar eclipse happens at least twice per year and can have up to five occurrences due to the Moon's alignment with the Sun and Earth.

The Moon casts two types of shadows: the umbra, a deep shadow where the Sun is completely blocked, and the penumbra, a lighter shadow where the Sun is partially blocked.

During a total solar eclipse, the Sun's faint atmosphere, the corona, becomes visible, presenting a stunning sight.

Baily's Beads, bright patches of sunlight streaming past the Moon's edge, are visible during a total solar eclipse.

Totality, the brief period when the Moon completely covers the Sun, lasts no longer than seven or eight minutes.

The Diamond Ring effect, a moment of intense sunlight visible as a ring around the Moon, marks the end of totality in a solar eclipse.

An annular eclipse occurs when the Moon is farther from the Earth, leaving a ring of light around the Moon instead of a total solar eclipse.

Viewing a solar eclipse without proper eye protection can cause permanent damage to the retinas, but a lunar eclipse is safe to view with the naked eye.

A lunar eclipse can be seen by anyone on Earth facing the Moon, unlike a solar eclipse which is localized to a specific area.

The red color of the Moon during a lunar eclipse is due to the Earth's atmosphere filtering out blue and green light, leaving only red light to illuminate the Moon.

Ancient Greeks used the size and shape of the Earth's shadow on the Moon during a lunar eclipse to estimate the relative sizes of the Earth and Moon.

The Moon is slowly moving away from the Earth by about 4 centimeters a year, eventually preventing total solar eclipses in about a billion years.

Lunar eclipses can be used to deduce the Earth and Moon's sizes, as demonstrated by the Ancient Greeks, who also inferred the Earth's spherical shape from the consistent circular shadow.

Transcripts

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

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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.