Say halo to the new year

Subscribers refract their light toward me

About this newsletter

Ooo, meta

Thanks to the planets aligning — literally — I got a big jump in subscribers last week, passing the 14,000 mark!

Last Monday morning I tweeted about the celestial lineup and the newsletter, and the tweet went kinda viral (well, viral for the kinds of things I put on Twitter, anyway). That in turn led to a jump in subscribers. So, to all the newbies, welcome! Glad to see you here! I hope you like it.

Did y’all see all the planets in the sky last week? I wrote about in the newsletter on December 26. I was able to catch some of it early on in the week before clouds settled in and we got snow; Venus and Mercury low in the west made a beautiful pair.

And hey, a reminder: Monday issues of this newsletter are and always will be free, but I also publish on Tuesdays and Thursdays for paid subbies. If you want more Bad Astronomy in your life, well, you know what to do.

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a description so you can grok it

On December 22, 2022, a wobble in the polar vortex dumped a huge parcel of frigidly cold air down across the middle of the US, dropping temperatures rapidly and mercilessly. Even for Colorado it got cold, and we went from about 10° C to -25° in a few hours. That happened overnight, so I knew when I got up that morning it was going to be rough.

As I was making my breakfast it occurred to me that this nasty cold air would probably freeze out any moisture in the air. And that meant lots of ice crystals floating around. And that meant…

Excitedly, I put down my cereal bowl and ran to a window facing east.

This is why I love knowing my science. I looked out the window toward the sunrise and saw this:

Yeah. It was very, very cool. The optical phenomena you can see in my photo are caused by ice crystals in the air bending (technically, refracting) sunlight. The different shapes and geometries of the crystals make the different effects.

And there’s a lot going on here. Faint but visible is a circle around the Sun called a 22° halo, caused by ice crystals shaped like a hexagonal prism in between you and the Sun. The light from the Sun is bent passing through the crystals, much of it at a 22° angle. So crystals 22° from the Sun in the sky bend light toward you, and you see a brightening. Any crystal 22° from the Sun does this, which defines a circle around it, hence the 22° halo. These are actually pretty common all year ‘round; I see a dozen or more a year at least, though few this nice.

There were also sundogs, technically called parhelia, the bright patches on either side of the Sun. These too are formed from light bending through hexagonal ice crystals, but in the case the crystals are flat, like plates. They fall through the air and naturally do so horizontally, face first. That means only the crystals parallel with the Sun in the sky (that is, directly to the left and right) can bend light toward you. That makes two bright spots: sundogs. Because the crystals are hexagonal, they bend the light the same amount as the ones that make a halo, so the sundogs lie along the arc of the halo, 22° from the Sun.

The horizontal lines streaking out of the sundogs are called parhelic circles. The same flat hexagonal crystals form them, but the light can reflect off the outside of the crystal, which means they can be at different angles and still send light toward you. That in turn means they can be well outside the sundogs but at the same angle of the ground. That defines a flat line, so you get those horizontal streaks.

At the top of the halo in that photo is a tangent arc. Again, hexagonal prism crystals are the source, but where the sunlight is bent twice: Once when entering the crystal and once when it leaves. The effects are more complicated and subtle, but can form a lovely bent V-shape on the 22° halo directly above the Sun.

You can barely see it in my first shot when the Sun had just cleared the horizon, but some minutes later when the Sun was higher all these effects actually got stronger:

Whoa. In this one you can see some color, too. The amount light is bent depends on its wavelength, which we see as color, so blue and red light are separated from each other due to the different refraction angles. You can see the right-hand sundog has red on the inside and blue on the outside due to this effect, which is very similar to how we get rainbows.

I love all this, for two reasons. One is the science, of course, because this is all chemistry and physics and optics and radiative transfer.

The other, of course, is because this stuff is staggeringly beautiful. It’s also a little weird. Rainbows are common enough, but honestly halos are maybe even more common; I’ve seen way more of the latter than the former (probably because it doesn’t rain much where I live, but we do get ice crystals in the air a lot). But the other effects — the sundogs, arcs, and more — are more rare, and so more precious when spotted.

And these are just the kinda basic haloes. There are way, way more kinds. If you love this as much as I do, then I strongly recommend the Atmospheric Optics website, which describes all this and more, has tons of photos, and even some interactive simulations where you can play with angles and such and see what haloes are created. It’s fun.

If you’ve never seen anything like these phenomena before, it may be because you live in a part of the world where they don’t happen as much. But, to be blunt, it may also be because you don’t look up enough. Most people don’t, and that’s too bad: There’s a lot of very cool stuff going on in the sky all the time. Clouds, bows, halos, stars, meteors, planets, and far more.

Look up! There’s a whole Universe shining down on you, and it starts just over your head.

Et alia

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!