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  • Have you ever seen an eclipse?

  • Fun, right?

  • Maybe even a little unsettling, but, you knew it was only going to last for a short time.

  • It must have been terrifying for early mankind.

  • Your faithful companion in the sky just up and disappears.

  • It would have felt like the world was ending.

  • Even now, with all we know, an eclipse is one of the most dramatic celestial events

  • you can see with the naked eye.

  • Were a little more blasé about astronomical events these days.

  • We use powerful telescopes to peer at the distant reaches of space, capturing not just

  • visible light, but all kinds of radiation - and we get back the most beautiful pictures.

  • We can see nebulae…... even supernovas.

  • But we still get pretty excited for eclipses that we experience firsthand,

  • right here on Earth.

  • You may be a little fuzzy on the details.

  • You know an eclipse has something to do with the Earth getting in the way of the moon -- or,

  • is it the moon getting in the way of the Sun?

  • Which is for a Solar Eclipse, and which is for a Lunar Eclipse?

  • Let’s take a closer look.

  • Sometimes, the Sun, the Earth, and the Moon all line up along a straight line.

  • When the Earth is in the middle, the shadow of the Earth can fall on the moon, which is

  • a lunar eclipse.

  • OR, in the other arrangement, with the moon in the middle, the shadow of the moon can

  • fall on the Earth, which is a Solar Eclipse.

  • Notice that these eclipses only happen during a new moon (in the case of a solar eclipse)

  • or a full moon (in the case of a lunar eclipse).

  • But why don’t these happen every single month?

  • You know that the moon orbits the Earth, and the Earth orbits the Sun.

  • But these orbits don’t take place in the same PLANE.

  • There’s a difference of about 5 degrees between the plane of the Earth’s orbit and

  • the plane of the moon’s orbit.

  • The two orbital planes intersect along what we call theline of nodes.”

  • This line passes through the Earth.

  • It’s only when the Sun, Earth, and Moon all line up juuust right along the line of

  • nodes, that you get an eclipse.

  • The plane of the Earth’s orbit is called the ECLIPTIC plane.

  • This name comes from the path we see the Sun take across the sky - theEcliptic”.

  • Of course, it’s actually the Earth moving around the Sun, but from our perspective on

  • Earth, it LOOKS like the Sun is moving in a fixed path across the sky.

  • This idea dates back to the ancient concept of a “Celestial Sphere.”

  • The Greek scholars of antiquity proposed the heavens consisted of concentric, crystalline

  • spheres, with Earth in the middle.

  • The Sun, the Moon, and other planets were each thought to have their own invisible sphere

  • that could freely rotate, and the distant stars were fixed in place

  • on a single distant sphere.

  • TheCelestial Sphereis still a useful way to picture our heavens.

  • To an observer on Earth, all celestial objects, no matter their distance, appear projected

  • on the inner surface of the Celestial Sphere - as if we were living under a dome.

  • Elements of spherical astronomy are still used today whenever we look for celestial

  • objects based on a particular date, time, and position on Earth.

  • Like eclipses.

  • During an eclipse, the Moon happens to land along the Sun’s apparent path across the

  • sky - theEcliptic.”That’s why we call it aneclipse.”

  • During a Full Moon, The Sun and the moon are on opposite sides of the Earth.

  • A lunar eclipse happens during a Full Moon if the moon is on the line of nodes and the

  • Sun is also on or near the line of nodes.

  • Now all three celestial bodies are in a straight line, and the shadow of the Earth

  • falls on the moon.

  • By contrast, the moon comes between the Sun and the Earth in a New Moon.

  • This is when a Solar Eclipse is possible.

  • These happen less frequently than Lunar Eclipses.

  • What’s more, it’s much harder for an observer on Earth to see a solar Eclipse

  • than a lunar eclipse.

  • A lunar eclipse is visible everywhere in the world - as long as it’s not cloudy.

  • If you can see the moon, you can see a lunar eclipse.

  • A solar eclipse, on the other hand, is only visible along a particular path - the path

  • of the shadow of the moon.

  • If you stand right in the center of the moon’s shadow (the umbra) you will be able to see

  • a total solar eclipse.

  • We call this the Path of Totality.

  • But if youre in the penumbra, this weaker, outer shadow, youll only see

  • a partial eclipse.

  • The farther away from the path of totality, the smaller the degree of eclipse youll

  • get to see.

  • There’s another reason why total solar eclipses are rare.

  • Remember that the Moon’s orbit is elliptical.

  • That means there are times when the moon is closer to Earth, and times

  • when it’s farther away.

  • The farther the moon is from the Earth, the smaller it appears.

  • When the moon is at its farthest from the Earth, we call it apogee.

  • Under these conditions, even if everything else is right for a solar eclipse, you can

  • see the Sun peeking out from around the moon.

  • We call this an ANNULAR solar eclipse, named for the ring of fire around the moon.

  • In these kinds of Eclipses, the Moon is so far away, its umbral shadow doesn’t reach

  • all the way to Earth.

  • But when the moon is closer to the Earth, that’s our chance for a total Solar Eclipse.

  • The moon looks bigger in the sky - big enough to cover up the Sun.

  • Let’s watch.

  • Now put on your safety glasses.

  • You don’t look right at the sun on a normal day, right?

  • That would permanently damage your eyes.

  • Well, the sun’s rays don’t change during an eclipse.

  • Get yourself a handy dandy set of eclipse glasses for everyone in your family.

  • They even make clip-ons for people who wear prescription glasses.

  • DON’T USE SUNGLASSES.

  • Theyre not the right kind of glass.

  • I’m dead serious here, people.

  • Get yourself some genuine eclipse glasses.

  • Well include a link in the description so you can get your own. These are mine.

  • Here’s what youll get to see during a Solar Eclipse if youre lucky enough to

  • be in the Path of Totality: Phase 1: Partial eclipse - put on your glasses.

  • Youre going to keep these on until the eclipse is at 100%.

  • No peeking.

  • In this first phase, the Sun is partially blocked by the moon.

  • Gradually the moon will move across the Sun’s disk.

  • This phase can last over an hour.

  • Phase 2: Baily’s Beads - keep your glasses on.

  • Did I mention you need to protect your eyes?

  • In this stage, the moon is surrounded by bright beads.

  • The appearance of this phase might surprise you, until you remember that the moon isn’t

  • perfectly smooth like a billiard ball.

  • It has valleys that allow the sun’s rays through.

  • This stage is named for astronomer Francis Baily, who was the first remarkable

  • to explain this phenomenon.

  • Phase 3: Diamond Ring - DON’T REMOVE YOUR GLASSES!

  • This phase is very, very short.

  • Baily’s beads disappear until there is one final bead left.

  • One last burst of sunlight through the moon’s valleys creates

  • what looks like a diamond ring.

  • These are the last seconds before the TOTAL SOLAR ECLIPSE.

  • Phase 4: Totality - this is the moment weve all been waiting for.

  • You can take your eclipse glasses off now for a short time.

  • The Moon completely covers the Sun.

  • We can see a little bit of the Corona peeking out - the Sun’s thin, outer atmosphere.

  • We never get to see this on a normal day, because the sun is so bright.

  • Notice what else we can see.

  • There’s Venus.

  • You can see stars.

  • You might even see some confused nocturnal animals here on Earth.

  • Observe all you can.

  • You only have a short time to experience a total solar eclipse, because of how fast the

  • moon’s umbra moves across the earth - around 1700 km/hr.

  • The MAXIMUM time for totality is about 7 and a half minutes, but that’s only when everything

  • is perfect - the Sun - Moon - Earth alignment and the distance between the Moon

  • and the Earth.

  • Usually totality is much shorter.

  • Phase 5: Final Stages - put your eclipse glasses back on.

  • Do it.

  • I mean it.

  • These last phases will look familiar - it’s repeating the earlier phases in reverse.

  • Diamond Ring, Baily’s Beads, and then we go through a long partial eclipse as the moon

  • moves back away from the sun.

  • Keep your eclipse glasses on from here on out.

  • Were coming back to normal life.

  • Everyone in the world gets many chances to see a lunar eclipse - usually a couple a year.

  • But Solar Eclipses are less frequent, and they often occur over water

  • rather than on land.

  • Some Solar Eclipse chasers will travel long distances, and even go out on boats far from

  • land to be able to witness this dramatic disappearance.

  • If you have the chance to see a solar eclipse, grab it.

  • But don’t forget these little beauties.

  • Thanks for watching Socratica!

  • Please share with your friends!

Have you ever seen an eclipse?

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什麼是日食?|| 日食||月食||天文學 (What are Eclipses? || Solar Eclipse || Lunar Eclipse || Astronomy)

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    林宜悉 發佈於 2021 年 01 月 14 日
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