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From dawn until dusk, that big ball of plasma we call the Sun reminds us just how brilliant it really is. But with all the technology at our disposal, our eyes might not be the best way to watch it work.

You've probably heard that you shouldn't look at the Sun, but like... always? What about when the Moon is smack dab in front of the Sun during a total solar eclipse? Surely that's the perfect time?

Well, no. Let's have Hank break down why you should never look at the sun directly, even when it's shielded during an eclipse.

Some people think that it's a myth that staring at Sun can make you go blind. It isn't. If you look at the Sun for too long, it'll do all kinds of damage to your eyeballs.

Sir Isaac Newton, legendary smart guy, learned that the hard way. He actually would wait in a dark room so his pupils would dilate, and then used one eye to look at a reflection of the sun in a mirror just to see if it would create some kind of cool afterimage in his vision when he looked away. And apparently he ended up seeing some lovely circles and colors, so he repeated the experiment two more times, and the so-called "phantasm" of lights and colors stayed in his vision for months.

His eyes did eventually go back to normal, but he ended up with a sunlight phobia. Good move, Newton!

But he wasn't the only one who ended up with vision problems because of the Sun. Giovanni Cassini, a 17th-century astronomer who studied Saturn, also complained of vision problems from observing the sun early in his career. Galileo must have experienced something similar, because he took to studying the Sun by shining a light through a pinhole onto another surface. And that is still one of the safest, simplest ways to observe the Sun.

Sunlight is mostly dangerous because of all of the ultraviolet radiation it has, and just a bit of that UV light can hurt your eyes. For example, it can give you photokeratitis, which is basically a sunburn on your cornea, the outermost layer of your eye. It causes blisters, pain, and inflammation-- not things that you want happening to your eyeballs.

Luckily, like regular sunburn, it's usually temporary, unless you get a really bad one.

Unfortunately, the damage doesn't stop at the top layer. Unlike your skin, your cornea is transparent and allows some of the UV light, called UVA radiation, to pass into other parts of your eye, and it can damage each of them in different ways.

After passing through your cornea, UVA light hits the lens, which bends and focuses light. Over time, repeated UV damage to the lens can cause cataracts, invasive tissue growths that make vision cloudy, and eventually, blindness.

Once it goes through your lens, which is also transparent, the UV light hits your retina, the structure at the back of your eye that transmits images to your brain. Normally, light stimulates the retina, which is basically a cluster of sensitive cells, to release signalling chemicals, but when those cells are overstimulated, like if you're looking directly at the sun, they put out way too much of that stuff. The signalling chemicals can actually damage the surrounding tissues, resulting in blurry, dark, or even lost vision, and that can be permanent. It's called solar retinopathy, and it's probably what Newton got.

After a while, UV light also tends to permanently damage a smaller part, right in the middle of the retina, called the macula. Along with some other things, it's responsible for the detail you can see right in the center of your field of vision. When bright light makes the pupils contract, any light that still enters the eye hits the macula.

Over time, that can lead to macular degeneration, which causes blind spots in the center of the field of vision. Like cataracts, macular degeneration can come from UV damage over time. Since we live in a world lit by sun, the most you can really do is wear UV rated sunglasses and avoid looking at anything too bright.

But also, in general, just never look directly at the Sun-- not through sunglasses, not through camera filters, and especially not through telescopes, binoculars, or other magnifying devices. Those things will concentrate the Sun's light and burn your eyes, which makes a lot of sense if you've ever seen someone focus sunlight through a magnifying glass to start a fire.

Even looking at the Sun's reflection could be bad. Some materials, like water, glass, snow, and sand, can be such efficient reflectors, the UV light bouncing off of them will still damage your eyes.

It's usually okay, though, to moongaze, since it's basically just a lit-up rock. Instead of being reflected, most of the light that hits the Moon is getting absorbed.

But you don't wanna look at the Moon when it's in front the Sun during a solar eclipse.

In normal sunlight, your pupils contract and your eyes make little random movements to protect themselves, but during an eclipse, most of the Sun's light is blocked, tricking your eyes into thinking that they don't have to do those things. Meanwhile, the part of the Sun that's not blocked is still emitting UV light, and your eyes are extra vulnerable.

This doesn't mean that you can never see the Sun, though. You just have to protect yourself first. Some people use welding goggles or solar filters, lenses covered with a thin layer of chromium alloy or aluminum, which blocks most of the light.

But even these can fail. Welding goggles aren't designed for sungazing, and solar filters can get damaged. The safest way to look at the Sun is probably still that 16th-century pinhole projector. Oh, Galileo! Four centuries later, and his ideas are still helping us out.

While the Moon isn't enough to protect your eyes from the Sun's harmful rays, a solar eclipse is such an awesome phenomenon. You have to sneak a peak somehow.

And here's some good news! By looking in the opposite direction, you could end up with less damage and just as much thrill. Here's Reid to explain how you can watch a solar eclipse by staring at the ground.

Solar eclipses tend to be pretty big news. The 2017 one in North America was hugely memorable for millions of people, including the SciShow team. And the 2019 one in South America was equally amazing and Instagram-worthy.

But during those events, the sky isn't the only thing people were looking at. They were also fascinated by these crescent-shaped lights that appeared on the ground, most noticeably underneath trees. The lights are one of the most fun parts of any solar eclipse, and the science behind them is pretty good, too.

The biggest thing causing these lights is called the pinhole camera effect. Like the name says, it was originally used to describe pinhole cameras, which work by letting light through a tiny hole in the screen. But the same physics applies when light from the eclipse passes through the spaces between tree leaves.

On the most basic level, light comes from the Sun, passes through those gaps, and then hits a projection surface, usually the ground. But the important part is that not every ray of light is able to pass through. The only rays that reach to the ground

are the ones angled in just the right way to make it through the spaces between the leaves.

And much of that light takes a very specific path. It starts from one side of the Sun, travels through the gap in the leaves, and hits the other side of the projection surface. Sothe light that comes from the left side of the sun ends up on the right side of the patch of ground, and vice versa.

This creates an image of the Sun that's both upside down and backward. And it's why those little blobs of light look so much like the solar eclipse happening way up in the sky. They're tiny images of the eclipse itself.

Technically, this means you can observe this phenomenon regardless of whether an eclipse is happening or not. It's just that the Sun without the Moon in front of it looks like a circle, so its projected image is just... a circle. Pretty nondescript. It's only once the Moon starts getting in the way that things become all interesting and pretty.

The cool thing about these images isn't just their shape, though. It's also the fact that they're really crisp and clear. Like, look at these lights compared to the blobs you normally see underneath trees. The normal lights are fuzzy, while the eclipse ones are sharp little moon-shaped things.

That happens for at least two additional reasons. One is that there's less ambient light during an eclipse. Normally, sunlight gets refracted and bounces around in our upper atmosphere. That gives the sky its nice blue glow, but that glow is another source of illumination, which makes the lights and shadows we see pretty fuzzy.

As a solar eclipse approaches totality and the Moon moves completely in front of the Sun, that additional glow is dramatically reduced. That means you don't get as much interference, so you get much cleaner, photo-worthy images.

The other reason these things get so sharp close to totality is because the visible portion of the Sun becomes narrower. Normally, the sharpness of a shadow depends on how far away you are from the light source. If you're really close to it, the shadow will be sharper, and if you're farther away, it will be more fuzzy.

But changing the size of the light source can have the same effect.

When you decrease the size of relative to the thing that's casting the shadow, like by covering up part of the Sun, the shadows change. The ratio of things that are totally in shadow versus only partially in shadow increases, and that gives those eclipse lights on the ground sharper edges.

So the next time you're experiencing a solar eclipse, take some time to look down. There's a lot that those funny-looking lights can teach you.

And hey, if you're not willing to wait for the next eclipse, you're not out of luck. At night, LED streetlights shining through trees can project some pretty cool pinhole images too.

So, ultimately, solar eclipses are cool in a lot of ways, but in this case, they help highlight some of the amazing phenomena that surround us all the time.

I hope you get the chance to check out these amazing shadows the next time you experience a solar eclipse, especially since looking at shadows is a great way to see what's happening without looking at the Sun. Because, in case you've forgotten, that's a really bad idea.

So, if you wanna check out the Sun, looking at the ground might be one of the safer ways to do it. But if you also want to see parts of the Sun that aren't visible from anywhere on Earth, you can look at the data captured by spacecraft. Here's Caitlin to describe one special mission which showed us what was happening at the Sun's poles.

Even though we've spent decades exploring the solar system, we've really only done it in two dimensions, generally speaking. But that makes sense. Since the planets are all on the same plane, called the ecliptic, we haven't needed to go way above or below the solar system. Everything we need to study is sitting nice and cozy in that plane.

Well, almost everything. From the ecliptic, it's almost impossible to study the area around the poles of the Sun, which, like the rest of our star, we're super interested in learning about. That is where the Ulysses mission came in. 

In 1990, NASA and the European Space Agency launched the first real orbiter to go out of the ecliptic, and it gathered tons of solar data for about 18 years. It revolutionized our understanding of our star, but there's a reason we haven't sent another one yet. Out-of-ecliptic missions are kind of a pain!

All the planets orbit in the same plane because they formed from the same big spinning disk

of stuff, and getting out of that plane is really difficult.

When you launch something from a moving body inside the ecliptic, like the Earth, your probe will automatically start traveling in a direction that keeps it inside that plane. To get out, you have to cancel all that motion and move basically perpendicular to where you started.

It's hard, and none of our rockets can pull it off right from launch. But Ulysses made it happen thanks to some cool engineering.

Usually, when we launch something that's gotta go pretty far pretty fast, we use a gravity assist. Basically, you get close to a large body, like a planet, then use its gravity to slingshot yourself along a new path with a higher speed. And that's true for out-of-ecliptic missions like Ulysses, too. You just have to get creative with it.

After leaving Earth, Ulysses went to Jupiter, which, as the most massive planet, can impart a huge amount of acceleration to anything that gets close enough. It flew up over Jupiter's poles, then let the planet's gravity sweep it over and back under itself. That got Ulysses going perpendicular to the ecliptic and flying back toward the point where it started, essentially going backward.

Then it started its big loop around the Sun. Getting Ulysses out of that plane allowed it to the study the Sun from a different angle, including its magnetosphere, composition, and solar winds.

But that's not all the mission did. It also carried instruments to study dust in the solar system and ones to look beyond the Sun at cosmic rays that come from really energetic sources like black holes.

And it was all totally worth it! Over almost two decades, Ulysses made nearly three full passes over the Sun, and it taught us a lot. Like, more than a thousand articles worth of stuff.

For one, Ulysses' unique perspective allowed us to make the first 3D survey of our star's magnetosphere and composition. It also took the first direct measurements of interstellar dust and showed that tons of it is flooding into the solar system; up to 30 times more than we thought.

And because Ulysses was active for so long, it managed to gather data over about one and a half solar cycles, which meant it could even see how the Sun changed over time. Every six-ish years, the Sun is either super active, with lots of flares and sunspots, or pretty quiet, and Ulysses showed that things like the solar wind, the flow of charged particles from the star,

actually change depending on its activity level.

It found that the solar winds are getting weaker in general, too. We think that coincides with the natural reduction in the Sun's magnetic activity, but we don't know just how, or if and when that trend will stop.

Either way, understanding these changes is super important to us on Earth, because the solar winds can damage electronics on the international space station and in satellites. Like, telecommunication satellites, which we need to tweet and, you know, call 911 sometimes. Because of Ulysses' discovery, we're better able to predict the behavior of the solar winds, so we're better able to protect astronauts in space and our precious, precious internet.

And looking much farther from home, Ulysses was also a player in confirming the existence of magnetars. These are a type of neutron star with super strong magnetic fields, which emit big doses of gamma radiation, kind of whenever they feel like it. At least, based on what we know now.

Along with a few other satellites, Ulysses snagged observations of only the fourth confirmed magnetar in 1998, one that was even brighter than the others. The magnetar hypothesis had just started to become accepted, so these measurements were really valuable to researchers.

And the list of Ulysses' discoveries goes on from there. The mission ended in 2008, and there are now mountains of papers and books, all using its data. So, even though getting out of the ecliptic was tricky, it was definitely worth it.

So worth it, that we're gonna launch another one. Today, the ESA and NASA are working on another out-of-ecliptic mission called Solar Orbiter. It'll launch in 2020 and orbit at an inclination of 25 to 34 degrees, which isn't as dramatic as Ulysses, but will still give us more exciting, 3D data about the Sun.

For now, though, scientists are still poring over that data from the first mission, so we'll have plenty to keep us busy until then.

In the end, it turns out you really can look at the Sun in all its glory... if you're looking at reconstructions based on data captured by space probes. Thanks to pinholes and space probes, you don't have to sacrifice your vision to witness a solar eclipse.

And if you want to know when the the next total solar eclipse will be so that you can look at the crescents on the ground, NASA has a page that explains where and when to watch. At the time of this video's recording,

the next one is due to hit North America on April 8, 2024.

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