I had noticed this on Google Earth and Celestia already.

Here's an image from the ISS: Terrestrial Terminator Line

As you can see on the horizon, at the terminator the atmosphere doesn't seem to follow the curvature any longer but instead moves away from the Earth's horizon. Is it just an optical effect or are certain atmospheric layers actually higher at the night side? What's causing this?


1 Answer 1


You are most likely seeing an effect of varying camera resolution, combined with nonuniform lighting caused by reflections off the clouds.

On any planet with an atmosphere, brightness dies not drop off all at once from day to night, but decreases gradually within a range along either side of the terminator (an effect we see as twilight or, in the reverse direction each morning, as dawn).

This dawn/twilight effect impacts what the camera can resolve of the atmosphere. As you approach the and pass terminator going from right to left, and the light of the sky dims, only the brightest parts are resolved and remain visible for a longer distance into the night. The layers of the atmosphere remain the same in reality but not in their lighting.

Now look away from the horizon. If you have clouds mixed with clear sky you probably know that when the Sun is rising or setting it tends to reflect brightly off the clouds; the reflections from the clouds are brighter than the surrounding sky (and the shadowed regions of those clouds).

Again, in your picture, you have slowly fading brightness as you go to the left and also clouds whose reflections have locally increased brightness. Apparently the clouds may be reflecting brightly enough for the camera to pick then up relatively far past the terminator, whereas the cloud-free areas get too dim at a shorter distance.

  • $\begingroup$ I don't think OP is asking about the reflections off the clouds, but instead the blue band of uninterrupted atmosphere at the horizon. $\endgroup$
    – Spencer
    Mar 12, 2022 at 16:20
  • $\begingroup$ @spencer please read my edit. $\endgroup$ Mar 12, 2022 at 20:37
  • $\begingroup$ But I've seen this on Google Earth and Celestia as well, that can't be due to resolution. The thing is, it's the layers themselves that seem to go away from the horizon, e.g. the troposphere usually looks yellow/orangeish at twilight and it doesn't curve on the terminator but goes away from the horizon. And below it you can see the Earth's shadow getting higher above the horizon the closer you are towards the night, the shadow "pushes" the troposphere up as if the troposphere's altitude varied between day and night. Is this just an optical effect or does the troposphere height actually change? $\endgroup$
    – Giovanni
    Mar 16, 2022 at 14:50
  • $\begingroup$ You write the layers remain the same but not in their lighting, but you still didn't make clear to me why. $\endgroup$
    – Giovanni
    Mar 16, 2022 at 14:52

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