Solar and Lunar Eclipses: Mechanics, Safety, and History

Name: Eclipses: Crash Course Astronomy #5
Uploaded: 2015-02-13T16:00:01+00:00
Duration: 10 min 31 s
Channel: CrashCourse
Description: Summary and key takeaways on Eclipses: Crash Course Astronomy #5: Summary & Key Takeaways, covering The Coincidence of Eclipses The Sun is about 400 times

CrashCourse

Feb 13, 2015

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10 min video

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YouTube video ID: PRgua7xceDA

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The Sun is about 400 times wider than the Moon and also about 400 times farther away, so both bodies appear roughly the same size in the sky. An eclipse happens whenever one object moves into the shadow of another.

Solar Eclipses

Mechanics and Shadow Types

A solar eclipse occurs when the Moon passes between Earth and the Sun. Because the Moon’s orbit is tilted about 5° relative to Earth’s orbital plane, the Moon usually passes above or below the Sun at each new moon, preventing an eclipse every month. When alignment does occur, the Moon casts two distinct shadows on Earth: the narrow, dark umbra where the Sun is completely blocked, and the broader penumbra where only part of the Sun is obscured.

Visual Phenomena

Inside the umbra, totality reveals the Sun’s corona, a thin outer atmosphere of gas that is normally invisible. At the edge of the Moon, sunlight streams through lunar valleys, creating bright spots known as Baily’s Beads—a phenomenon first described by Francis Baily in 1836. As the Moon moves off the Sun’s face, the Diamond Ring effect appears, a brief flash of brilliance from the last bead of sunlight.

Annular Eclipses

When the Moon is near the far end of its elliptical orbit, it appears smaller than the Sun and cannot cover the solar disk completely. The result is an annular eclipse, leaving a bright ring of the Sun visible around the Moon’s silhouette.

Safety Precautions

Looking directly at the uneclipsed Sun can cause retinal damage. The greatest danger during a solar eclipse is the flash of sunlight immediately after totality, when pupils are still dilated. Safe viewing requires approved solar filters; homemade filters such as CDs, DVDs, or old camera film are ineffective and risky. Lunar eclipses, by contrast, are safe to watch without any protection.

Lunar Eclipses

Mechanics and Visibility

A lunar eclipse occurs when Earth blocks the Sun, casting its shadow onto the Moon. Anyone on the night side of Earth who can see the Moon will witness the event. Because Earth’s shadow is much larger than the Moon, a total lunar eclipse can last nearly two hours.

Coloration and Atmospheric Effects

During totality the Moon often turns a deep red. Earth’s atmosphere scatters blue and green light, allowing only red wavelengths to pass through and illuminate the Moon. This process projects the combined light of all global sunrises and sunsets onto the lunar surface.

Historical Significance

Ancient Greek astronomers observed the circular shape of Earth’s shadow on the Moon and used it as evidence that Earth is a sphere. This early use of eclipse observations helped shape the foundational understanding of Earth’s shape and size.

The Future of Eclipses

The Moon is receding from Earth at roughly 4 cm per year due to tidal forces. In about one billion years, this gradual drift will make total solar eclipses impossible because the Moon will appear too small to completely cover the Sun.

Takeaways

The Sun and Moon appear the same size because the Sun is roughly 400 times wider and 400 times farther away than the Moon.
Solar eclipses require a precise alignment of Sun, Moon, and Earth, and the Moon’s 5° orbital tilt limits them to at least twice a year.
Total solar eclipses reveal the Sun’s corona and produce phenomena such as Baily’s Beads and the Diamond Ring effect.
Viewing a solar eclipse safely demands certified solar filters, while lunar eclipses can be watched without protection.
Earth’s atmosphere scatters blue light during a lunar eclipse, giving the Moon a red hue that represents the combined light of worldwide sunrises and sunsets.

Frequently Asked Questions

Why does the Moon appear the same size as the Sun during eclipses?

The apparent size match occurs because the Sun is about 400 times wider than the Moon but also about 400 times farther away, making their angular diameters nearly equal. This coincidence allows the Moon to cover the Sun completely during a total solar eclipse.

What causes the red color of the Moon during a total lunar eclipse?

Earth’s atmosphere filters out blue and green wavelengths, letting only red light pass through and reach the Moon. The red light, which is the combined illumination from all global sunrises and sunsets, gives the eclipsed Moon its characteristic blood‑red hue.

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Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Iso Certified Solar Eclipse Glasses Recommended

These glasses provide the necessary protection to safely view the sun during a solar eclipse, preventing permanent retinal damage.

Amazon →

Astronomy Telescope For Beginners

A telescope allows for detailed observation of lunar eclipses and the moon's surface features mentioned in the lecture.

Amazon →

The Backyard Astronomer's Guide

This book provides comprehensive guidance on observing celestial phenomena, including eclipses and orbital mechanics.

Amazon →

Solar System Model Kit

A physical model helps visualize the orbital mechanics, tilt, and shadow casting described in the eclipse lecture.

Amazon →

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Full Transcript YouTube

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.
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