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SYFY WIRE Bad Astronomy

Time-Lapse Video: Stars That Fell As Snow

By Phil Plait

I do enjoy watching the odd time-lapse video of the sky or two, and I love sharing them with y’all as well. Many are beautiful, of course, and that’s reason enough to spread the word. They encourage people to go outside and look up, and I hope by now you know how I feel about that!

But these highly sped-up animations show us the motions of the sky we miss in our 60-seconds-to-the-minute lives. It’s only when the heavens are accelerated that some things pop out, and it’s worth noting them.

So pay attention, folks! Watch this video by Lithuanian photographer Tadas Janušonis (whose work I’ve featured before), titled “Stars That Fell As Snow.” There will be a quiz after.

Lovely, isn’t it? You can see halos around the Moon, light pillars, and other beautiful phenomena. (I love the snow on the ground sparkling and twinkling as the flakes reflect the moving moonlight.) But that’s not what I want to bring to your attention.

At 1:31, a sequence starts showing Orion sliding past the silhouette of a house. It’s very nice, but there are two hidden secrets in it. Did you see them?

Let me help. To the left of Orion and near the top of the frame is the bright star Procyon (the brighter star Sirius is below it—at the top of this post is a picture from the video, and Procyon is the star at the top left). Restart the video at 1:31 and keep your eyes on Procyon as it moves left to right.

Did you see it that time? There is what looks like a star just below Procyon, and it suddenly appears, gets brighter, then fades. Replay it again if you need to. Did you notice anything else odd about it?

The “star” doesn’t move with the other stars! It seems to sit tight in the sky. That’s because it’s not a star, it’s a geostationary satellite. These are satellites launched into special orbits that are about 36,000 kilometers (22,000 miles) up. At that height, they orbit the Earth once every 24 hours, the same as the rotation rate of the Earth. Because of that, they appear to stay more or less glued to the sky, while the stars slide past them (from their point of view the Earth looks like it’s not spinning below, so they’re always over the same spot on our planet, making them useful for weather observations and communications).

I suspect the satellite gets brighter and fades because it’s reflecting either sunlight or moonlight off its solar panels, and as the angle changes we see that reflected light pass over us. Usually they’re pretty faint and hard to see.

I mentioned there’s a second secret: It’s a second geostationary satellite! Just before the bright one appears under Procyon, there’s another one to the left of it, at the tip of a tree branch. At first I thought it was the branch, maybe reflecting some ambient light. But after watching it a few times I’m pretty sure it’s another satellite.

Incidentally, these satellites generally have orbits that are very nearly directly over the Equator. Both of the objects in the video are in fact in the part of the sky corresponding to that position (the “celestial equator”) so that adds credence to my hypothesis.

And there you go! Like I said, in a time-lapse video there are things that can appear and make themselves known that you would otherwise be totally in the dark about. Hidden away in the sky, there’s knowledge and beauty and science and fun.

But that’s redundant, isn’t it?

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