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SYFY WIRE Bad Astronomy

Sunset Rocket Launch Casts a Long Shadow

By Phil Plait

On May 16, 2014, an Air Force GPS satellite was launched into orbit on a Delta 4 rocket from Cape Canaveral, Florida. Launch photographer Ben Cooper was there with a GoPro wide-angle camera, and captured the launch on video:

Pretty cool. You can see the engines ignite at about the two or three second mark, and then it takes about eight or so seconds for the sound to be heard (though with the background noise it’s hard to tell). From that I’m guessing he was roughly 2-3 kilometers away; sound travels at a kilometer every three seconds. Incidentally, the clicking sound you hear is another camera taking pictures.

The launch was at sunset, just before the Sun went below the horizon. Cooper reports the weather was quite clear and humidity low, though I think the definition for “low humidity” in Florida is not on the same scale as where I live in Boulder, Colorado  (native Floridians have been known to desiccate and blow away in Chinooks here).

However, there was enough haze to create a pretty cool effect: Look at the plume of smoke from the rocket as it rises. Near the ground it’s dark, but as it gets higher it gets brighter. That’s because near the ground it’s lit by the low Sun, which has some of its light blocked by atmospheric haze. As the rocket gets higher, the Sun appears to be higher off the horizon from the rocket’s point-of-view, and it gets brighter. In fact, about a minute into the video you can see the shadow of the plume extending off to the right (roughly east-southeast; this time of year the Sun sets north of west)—that shadow is cast on the haze in the air itself!

Note too the change in color of the plume as it goes up, from dark orange to yellow. That too is caused by the Sun’s light. When the Sun is near the horizon, junk in the air scatters away bluer light, letting only red light through. This effect is smaller the higher the Sun is off the horizon, so if the air is murky you see the Sun turn yellow, orange, then finally red as it sets.

The rocket sees this in reverse as it gets higher, so you see the plume color change with height. Cool!

I’ll leave you with this: Another photographer, Matthew Travis, also had a GoPro camera … which he placed in the fire trench just a couple of hundred meters from the rocket! That video is something (turn your speakers up and set it to HD for the full effect):

I’ve seen a handful of launches in the past (A Saturn V, a Delta with nine strap-on boosters that we saw fall away, and a Space Shuttle), and they are every bit as cool as you imagine they would be. I hope to see more in the future. Someday I’ll watch a Falcon 9 (or a Falcon Heavy …) go up. That would be a lot of fun.

Until then, I’ll just peruse Cooper’s amazing Launch Photography picture gallery. You should, too.

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