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Bright pillars of light... clouds of glowing dust... shimmering discs floating around the sun... signs of extraterrestrial activity, or is there a more rational and scientific explanation for this phenomenon?

Thumbnail image: Christoph Geisler

Hosted by: Michael Aranda
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Sources: (Mix of Everything) (Mix -- Many photos!) (Mix of everything but throughout many pages) (Diamond Dust) (Sun Halos -- more science-y) (Sun Halos -- more blog-y) (Sun Dogs -- sweet diagram in this one) (Sun Pillars) (Sun Beams -- SciShow) (Temp. Reference)
Michael: Lots of things get mistaken for UFOs. Sometimes, it's a Frisbee, sometimes it's the planet Venus, sometimes it's a military missile, and sometimes what looks like a UFO might even be the effects of the Sun. Sunlight can make all sorts of optical illusions: from big pillars that look like abduction beams, to huge halos in the sky. And most of these optical illusions happen for similar reasons: because of the way sunlight interacts with small ice crystals in the atmosphere

Normally, you'll mainly find these tiny crystal only at high altitudes where it's cold enough for water vapor in the air to freeze into small crystals. But if it's cold enough outside - usually below ten degrees Celsius - these ice crystals can form lower down where they can start playing tricks on your vision. And if it's really cold, there can even be a whole cloud of ice crystals which is kind of an optical illusion all by itself.

It's sometimes called Diamond Dust because it looks like a sparkling cloud. The sparkling comes from the ice crystals reflecting and refracting the light that hits them. Light that bounces off the crystals is reflected, and light that goes through the crystals and gets bent along the way is refracted.

This reflection and refraction can also lead to other optical illusions depending on the size, orientation, and location of the ice crystals. The most common illusion is the Sun Halo, or more specifically: The 22 degree Sun Halo. Which gets its name because the halo is almost always seen 22 degrees offset from the Sun.

That's a little less than the distance from the tip of your thumb to your pinky if you stretch your hand out at arm's length with your fingers spread out. Twenty-two degrees might seem like kind of a random number, but it has to do with the property of ice called its refractive index. That refractive index.

Every material has its own refractive index, which is a measure of how much the material bends the light that passes through it. Regular air has a refractive index of almost 1, meaning it only bends light very slightly when it enters the atmosphere from the vacuum of space. But ice has a refractive index of about 1.3. That refractive index means the light bends at a certain angle when it enters the ice And then it bends again when it leaves the ice.

The total effect is about a 22 degree bend Though the angle changes slightly depending on the color of the light and how the light hits the surface of the crystal. If you have red light, for example, which has a longer wavelength, then it won't bend quite as much as violet light, which has a shorter wavelength. Sometimes these different angles make the halo look kind of like a rainbow around the sun.

The angle also depends on how the light hits the crystal's surface. The crystals are arranged pretty randomly. So some will refract light a little less than 22 degrees And some will refract light a little more than 22 degrees, which makes the halo look kind of thick and blurry. Sometimes though, the crystals do line up with one another.

As they float down through the atmosphere the aerodynamics of the crystals' shape makes them slowly rotate until they're horizontal, flat compared to the horizon. And that's when you'll start to see an illusion known as Sun Dogs. Which show up much lower in the sky.

Sun Dogs are a lot like Sun Halos, just much more intense. Since most of the crystals are lined up in the same direction, you can see much brighter spots because they're all bending the light in the same way. If the crystals are especially big they tend to be disturbed by the atmosphere a little bit more, so they tilt and wobble. This means that the crystals are at slightly different angles, so there are more possibilities of how the light will hit the surface of the crystal. They're still aligned enough to concentrate the light but the Dogs will be bigger and blurrier.

Another optical illusion you might see is a Sun Pillar. A Sun Pillar This one relies on reflection instead of refraction and you normally see it around sunset. Like with Sun Dogs, the ice crystals are arranged roughly horizontally in the sky, but this time they actually act like a giant mirror that reflects the light from the sunset back down toward Earth. The crystals can be very large Especially as it gets colder in the evenings. And again, like with the the Sun Dogs, these large crystals can become tilted at different angles so they reflect light at different angles. You end up with a long column of reflected light: a Sun Pillar.

Sun Pillars might look kind of like sunbeams but, sunbeams come from light passing through lots of dust particles, which scatters the light, whereas sun pillars are created by these ice crystals. So if you see rays of sunlight that seem to be shining from weird places, it's probably just the ice in the air. One thing it's definitely not, is an alien abduction beam.

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