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SYFY WIRE Bad Astronomy

His First Halo

By Phil Plait

Swedish astrophotographer Göran Strand took that picture above. It shows a halo around the Sun, replete with parhelia, over Lake Storsjön. That’s his son in the photo.

Besides being extremely beautiful and poignant, it’s also just an astonishing shot. Strand used a 14mm wide-angle lens, which he needed because the full halo is 44° across, a quarter of the way around the horizon. A lens like that really compresses distance, so I’d guess his son was standing right in front of him.

Halos likes this are relatively common. They’re due to flat hexagonal ice crystals in the air. As sunlight enters a crystal it gets bent, refracting as it enters and as it leaves the faces of the crystal. The light gets bent by a total of about 22°, so crystals that distance from the Sun along your line of sight bend the sunlight toward you. Crystals closer to the Sun bend the light away from you so it’s darker inside the halo. Depending on the exact orientation of the crystal, some of the light gets bent more than 22°, so ice outside that 22° bright ring also bend light toward you, though not as strongly, so the halo fades away outside the optimal 22° angle.

When I say “degrees”, I mean an angular distance on the sky, where 90° is the angle between the horizon and the zenith, straight overhead. Your outstretched fist is roughly 10° (I have big hands, so for me it’s more).

The light forms a circle because that shape defines a constant distance from a point; for a halo that point is the Sun (the center of the circle), and the ice crystals 22° away from it fall along the circle of that size.

As it happens, red and blue light get bent by different amounts as they enter and leave the ice crystals; red light is bent a wee bit less, so the inner edge of the halo is redder and the outer part bluer. You can see that in Strand’s photo.

Parhelia—also called sundogs—are also caused by the hexagonal crystals. As the crystals fall through the air they align flat, face down to the ground. When the line between the Sun and crystal is parallel to the horizon, a lot more light gets bent toward you, so you get really bright spots on the halo on opposite sides.

It looks like he has a sun pillar in there, too: a vertical shaft of light caused when the flat crystals are slightly tipped, reflecting light toward the observer. That can only happen with crystals seen directly above or below the Sun.

I’ve seen halos and parhelia I-don’t-know-how-many times. Dozens certainly. Hundreds? Maybe. I look up a lot. If you haven’t ever seen this incredible and lovely display in the sky, then you know what to do.

And check out more of Strand’s photography. I’ve featured a lot of his work on this blog, and you’ll see why. 

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