字幕表 動画を再生する 英語字幕をプリント 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. We’re 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 the “line 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 - the “Ecliptic”. 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. The “Celestial Sphere” is 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 - the “Ecliptic.”That’s why we call it an “eclipse.” 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 you’re in the penumbra, this weaker, outer shadow, you’ll only see a partial eclipse. The farther away from the path of totality, the smaller the degree of eclipse you’ll 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. They’re not the right kind of glass. I’m dead serious here, people. Get yourself some genuine eclipse glasses. We’ll include a link in the description so you can get your own. These are mine. Here’s what you’ll get to see during a Solar Eclipse if you’re lucky enough to be in the Path of Totality: Phase 1: Partial eclipse - put on your glasses. You’re 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 we’ve 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. We’re 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!