The Great American Eclipse (Script) (Patreon)

This is a community service announcement. If you’re captured by a tribe of cannibals on August 21st, this year, please remember to raise our hand to the heavens and threaten to block out the Sun. Get the timing right and the sky will blacken, the stars will emerge, and you’ll be declared a god. Oh, it’ll only work if that cannibal tribe is located somewhere in a narrow band across the continental United States. Yup, the U.S. is going to witness its first total solar eclipse in forty years. 

Solar eclipses are rare and special events. Today I want to talk about why they’re so cool, and what you need to know in order to see this one. First, let’s go over the two eclipse types: A Lunar eclipse is when the Earth’s shadow falls on the Moon. A solar eclipse is when the Moon’s shadow falls on the Earth. The Moon has to be on the opposite side of the Earth compared to the Sun to catch the Earth’s shadow, and so lunar eclipses are always during full moons. And the Moon has to be between the Sun and Earth for its shadow to hit us, so solar eclipses are always during new moons.

But there’s a New Moon every month. Why the forty-year wait for an American Solar eclipse? Two things: alignment and shadow size. The Moon’s orbit about the Earth and the Earth’s orbit about the Sun--the Ecliptic plane--are misaligned by about 5°. Most months, the Moon’s shadow misses Earth. Only when the Moon crosses the Ecliptic plane during a new or full moon can an eclipse occur. That alignment happens about twice a year, and usual results in one solar and one lunar eclipse. Sometimes you even get three eclipses. The solar eclipse on August 21st will be preceded by a partial Lunar eclipse on August 7th. It’ll be partial because the Earth won’t completely cover the Sun from the Moon’s perspective. Everyone who can see the moon will see that lunar eclipse. However to see the solar eclipse you need to be in the narrow path of the Moon's shadow. This time, that path crosses the united states.

The fun starts at 9:04 a.m. Pacific time when the edge of the Moon’s shadow reaches the west coast. That’s the penumbra of the moon’s shadow, where the Sun isn’t completely blocked. Let’s say you’re standing on an ocean cliff in Depoe Bay, Oregon, because that’s where the umbra – the total eclipse – first makes landfall. At first you won’t notice much change in the glorious sunny day … fingers crossed for that last part. But something about the light seems clearer, the shadows, sharper. Take a walk through the pine woods behind you and you might notice the dappling of light between the shadows have changed. Usually they’re circular. Now there’s a bite taken out of them. In fact, the spaces between the leaves act like pinhole cameras, projecting the Sun’s image onto the ground. In those dapples you’re witnessing the beginning of the eclipse. This is where you might set up your own pinhole camera to watch on oncoming eclipse. There are plenty of resources online for how to make one. Whatever you do, don’t look directly at the Sun! Unless you have highly specialized protective glasses. No joke. You need this, and sunglasses won’t cut it. You need aluminized Mylar, or number 14 welder’s glass, something approved specifically for viewing the Sun. The easiest way is to search for eclipse glasses online.

Over the course of the next hour, the Moon eats further and further into the Sun. You start to realize the two objects are the same size in the sky. This is really pure chance. The Moon is much smaller but much closer than the Sun, and it just so happens that they have the roughly same angular size on the sky. This August the Moon will span 0.538°, just big enough to completely obscure our 0.527° Sun. It isn’t always so perfect. The Moon’s orbit is elliptical, and so sometimes it eclipses the Sun when it’s a bit further away. It appears smaller to us, and so doesn’t completely clock the Sun. Such eclipses are called annular eclipses, because they leave a ring of bright sunlight around the Moon’s disk. They’re OK I guess, but August will give us a total solar eclipse. Those are incomparable. Enjoy it while you can. Every year the Moon steals a little bit of Earth’s rotational kinetic energy and drifts about 3.8 cm away from us, and so in the future won’t block out the entire sun. In only half a billion years, the Earth will have its final total Solar eclipse.

As partiality deepens, the shade by the trees becomes full of bright crescents. Now there’s definitely something off about the light. It’s eerie. Everything looks like a high-contrast Instagram filter. We’re nearing totality. With your glasses on you see the Sun is a mere sliver--and it’s shrinking. The sky’s blue deepens, a breeze stirs, and the temperature drops. Thin shadows ripple unpredictably on plain surfaces. These are called shadow bands, and they’ve perplexed scientists for a long time. They may be an atmospheric effect, perhaps from the same turbulence that makes stars twinkle but now it’s the last thin sliver of the Sun that is twinkling. The stars, by the way, start to come out. The Sun’s final crescent contracts to a last spark on one side of the Moon, like a diamond ring, and then it’s gone.

Now you can take your glasses off! It’s the one time you can stare safely straight toward the Sun. A lot happens at once. For probably the first time in your life you see the chromosphere, red from a specific electron transition in the hydrogen of the Sun's upper atmosphere. Below it you see Baily’s Beads sparkling around the rim of the Moon. Really they’re extra sunlight peaking through Lunar valleys. They fade quickly. What shows up now is the ghostly light of the corona, the Sun’s outer atmosphere. The corona is always there, but it's faint glow is normally overwhelmed by the glare of the sun's surface.

Right next to the blocked Sun you’ll see the bright star Regulus and its constellation, Leo.  A little further away you’ll see the planets Venus, Mars, Jupiter, and the elusive Mercury. Mercury orbits so close to the Sun that it’s hard to catch in a dark-enough sky. Now it will shine almost as brightly as the Mars. Further out still, you’ll see Orion. He’s not normally up in the summer. Nice to see him a few months early.

Totality will reach Depoe Bay at 10:16am Pacific time and last just under two minutes. That central umbral shadow is about 110 km or 70 miles across, and it will race eastward at about 2000 mph. Stand on a mountain along the path, and you’ll see the umbra racing towards you. From Oregon the eclipses heads through Idaho, Montana, Wyoming, Nebraska, Kansas, Iowa, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina, before finally slipping into the Atlantic at Cape Romaine, South Carolina at 2:49pm Eastern Time, an hour and a half after it first made landfall.

As totality fades, Leo, Orion, and the corona disappear in midday twilight. Baily’s Beads and the red chromosphere flash along the edge of the Moon, the diamond ring emerging on the western side. The shadow bands scatter, and the bright blue sky returns like nothing even happened.

So what do you need to do to make sure you catch the Great American Eclipse?

Google the path of totality and find a good spot along that narrow strip stretching between Oregon and South Carolina. That includes looking up typical August weather for candidate spots. You need clear skies. Have a backup plan in case of clouds, and of course be check the weather forecast from several days earlier. Also, remember that everyone else will be chasing the eclipse too, so book hotels early – honestly, it’s already too late for some places. Alternatively find somewhere way off the beaten track. Most importantly, sort out serious eye protection like I already discussed.

Look, I can describe a Solar eclipse, but you really have to see it for yourself. Astronomical event of this caliber will change your life. I saw my first on Flinders Island off the south coast of Australia when I was a kid, and it really enhanced my love of astronomy. Who knows, without it I may never have ended up doing a show like Space Time.