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In this video of sunrise at Cadillac Mountain, Maine, the landscape closer to the horizon appears to brighten sooner than the foreground. It makes sense to me, the terrain at the horizon is experiencing the sunrise before the terrain at the water level in the foreground. I'm wondering what is the minimum height someone would have to be see the difference in the brightness on the ground in the distance to the brightness on the ground nearby.

For example, if from 200' high one can see at least 15 miles to the horizon, and the sun rises over a minute earlier 15 miles away from the observer, would the observer be able to see a difference in the brightness of the two locations?

I'm assuming a latitude of 50°N.

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  • $\begingroup$ Maybe more suitable for Astronomy SE? $\endgroup$
    – Jean-Marie Prival
    Commented Apr 12, 2021 at 20:43

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TL;DR: It certainly is possible from a geosynchronous orbit, an altitude of 35786 km. It is also possible from a very high altitude weather balloon.

A key issue is that the terminator is not a line. Thanks to the Earth's atmosphere, the terminator is instead a band that is about 1000 km across that gradually transitions from sunlit to dark. At most latitudes sunlight notably starts to decrease at least 10 minutes before sunset, but one can continue with outdoor activities for a half hour or more after sunset. (The reverse is true for sunrise.) That 40 minute period or longer time span corresponds to a very wide band. Even an SR-71 could barely have seen that ~1000 km wide band that transitioned from sunlit to dark.

In addition to barely being high enough, another issue with an SR-71 was that they moved so fast. Something that flies even higher than did an SR-71 but flies much, much slower than an SR-71 is needed. That would be a high altitude weather balloon. A high altitude weather balloon (30 km or higher) could see both the bright and bright sides of that 1000 km band, and it could see that band slowly moving.

Even higher up (much higher up), that widish band doesn't look at that wide from the perspective of a geostationary satellite. Now it does looks a bit like a line. Just as is the case with a geostationary high altitude weather balloon, a geostationary satellite would be able to see the terminator (now a line rather than a band) move over the more or less stationary Earth.

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  • $\begingroup$ Stealing from Rod Serling, "You're traveling through another dimension -- a dimension not only of sight and sound but of mind. A journey into a wondrous land whose boundaries are that of imagination. That's a signpost up ahead: your next stop: the Twilight Zone!" $\endgroup$
    – David Hammen
    Commented Apr 13, 2021 at 2:47
  • $\begingroup$ "I'll be back!" - The Terminator $\endgroup$
    – f.thorpe ♦
    Commented Apr 13, 2021 at 4:22
  • $\begingroup$ @DavidHammen Yes, I realize terminator was a poor choice of words. I'll edit my question. $\endgroup$
    – Bob516
    Commented Apr 13, 2021 at 4:34

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