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SYFY WIRE Bad Astronomy

AMAZING Halo Display

By Phil Plait

Over the weekend I posted an amazing and lovely picture of a sun halo, taken in Sweden. That prompted Kalle Centergren, a self-proclaimed science nerd to send me a photo he took of a halo he saw skiing in Austria.

sun halo

That’s the photo above, showing an incredible array of optical effects: The 22° halo, parhelia, tangent arcsparhelic circle, a supralateral arc, a Parry arc, a circumzenithal arc, possibly a 46° halo, and probably others I’m not even sure of. That is one of the craziest and most vivid displays I’ve ever seen.

But it gets better: Centergren took video, including some using a drone:

Seriously, right? I expect there was a lot of small snow crystals in the air, suspended over the slopes, creating all those ridiculously beautiful effects.

I went skiing once and tore my ACL. Centergren goes skiing and gets some of the most incredible halo footage I’ve ever seen. Seems fair.

Also, this made me laugh: I noticed that in the drone footage there’s a lot of what looks like detector noise; white specks that flash all over the field. I'm used to seeing this in astronomical data taken right off the telescope; they're caused by cosmic rays, little particles zipping though space that slam into the detector and deposit their energy behind. They’re a massive pain when you’re trying to observe faint objects; you have to go through a lot of careful processing to get rid of them.

So of course that was my first thought when I saw the footage; it's what I'm used to. But that's not at all what those flecks are: They're actually snow crystals blowing around in the air, the very same crystals causing the halo and other optical effects! They appear bright because they're near the camera, and flashing in the sunlight as the camera looks toward the Sun. 

There's an old expression in skepticism: When you hear hoof beats, think horses before you think zebras. In this case, my brain is trained to listen for zebras!

It goes to show that your brain can be easily fooled, even when nothing is really trying to fool you. It just happens. That's always worth keeping in mind.

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