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    Re: Observe refraction: Earth's Limb from the ISS
    From: Frank Reed
    Date: 2016 Apr 5, 12:50 -0700

    David Pike, you wrote:
    "I'm afraid my untrained eye is unable to spot any jumps, but I would rather like to know what the flashes are on the Earth. "

    The flashes are just lightning. If you watch closely, you can see that they're associated with clouds. Since these flyby videos are speeded up by a factor of 20 to 30, the lightning flashes seem very rapid. Here's something fun: if there were air between ground and the space station altitude, the astronauts could wait for the thunder and determine their altitude, right? Let's see... at five seconds per mile to travel the 240-ish miles from ground to station... sound would take 20 minutes! So by the time the sound of the thunder arrived, the space station would already be almost a quarter of the way around the globe. :) Needless to say, the sound of thunder can't propagate much beyond the lower stratosphere, far, far below orbital altitude.

    Back to those "jumps" that seem to occur in the video that Brad Morris linked. They're definitely an illusion. If you watch frame-by-frame, they evaporate. And they couldn't be real. This is way to high up for refraction... I double-checked the altitude of that green airglow level: it's just above the top of the mesosphere. This is a "no man's land" in the atmosphere: no spacecraft can orbit (air resistance, slight though it is, is high enough to bring a spacecraft down in much less than one orbit at this height), and no aircraft or balloons can fly at this height. It's the domain of suborbital "pop-up" missions and sounding rockets, about 5-10km below the international aeronautical lower limit of space, defined as 100km above the Earth's surface. But as far as refraction is concerned, there is no air here. I previously estimated an atmospheric density 100,000 times lower than sea level. Since the correct altitude is somewhat higher, it's actually about one million times lower density than at sea level. At these altitudes, atmospheric density drops off by a factor of 10 for roughly every 16 km (nearly 10 miles) increase in altitude. Refraction is exceedingly slight, and change in refraction --which would be required for those jumps to be real-- is unmeasurable.

    Something else to contemplate: when the astronauts photograph that green airglow layer, they're looking down from an altitude of 400km at a surface that is a relatively constant spherical altitude of about 95 km above the Earth. So how far away is that airglow horizon? And how far away is the "true" terrestrial horizon? Just convert the altitudes to feet and then apply the usual square root of height of eye rule. That'll get you within ten percent. By my estimate, the true horizon is roughly twice as far away as the true terrestrial horizon.

    Frank Reed

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