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SYFY WIRE Bad Astronomy

Arc of Truth

By Phil Plait

I was out strolling recently on a cloudy day, and—as I always do—I took a quick look around the sky to see if anything interesting was to be seen.

I do so love that habit. This particular time, I was greeted with a most unusual sight: a rainbow segment, just a small arc levitating in the clouds. It was raining in that direction, so there were raindrops in the air, a critical component to make a rainbow.

But the other half of that recipe is sunlight, and that day was almost completely overcast. Almost. In the west there was a small break in the clouds, enough to let a single shaft of sunlight through. That illuminated the suspended raindrops, refracted, and created the partial bow.

What I love about this, though, is that you can see that shaft of light! Haze, raindrops, and other particulates in the air scattered that light, reflected it back to me, lighting up in the path of that sunbeam, contrasted with the storm clouds behind it. And you can tell that the opening in the clouds to the west wasn’t fully open; there must have been a small cloud in the middle to account for the shadow ray piercing through the light shaft and arc.

Mind you, there was no other hint of a rainbow anywhere else in the sky. That was it, the only clear sunlight available. And it was only those raindrops, seen at just the right angle, that allowed me the view of the broken spectral arc. Someone a kilometer away in the wrong direction wouldn’t have seen anything at all. I was at the right place at just the right time.

But even then, it all would’ve been for naught had I not looked up.

Oh, that sky we live under. Wonderful, isn’t it? You should look at it more often.

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