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Some years ago I had the chance to climb the Himalayas. Not the Everest of course, but the Annapurna Base Camp which is about 4km above sea level.

During the night of the stay I went out to do some stargazing and I found that the night was very bright, so bright that I was able to make out letters on a book. (Of course I didn't read.) I figured that since there was practically no light coming from the ground within several kilometers to illuminate the air, the light from the moon and the stars were comparatively bright (just the opposite of why we can't see stars clearly from a city).

But recently it hit me that, well, night is supposed to be dark. Anyone who lived outside cities will tell me that. Lack of light source from the ground will make the moon appear brighter because there will be greater contrast but it won't make moonlight reaching the ground any brighter, will it? So this confused me. How was that night in the Himalayas so bright? Is it an expected thing if I'm so high up, or are there some factors that might have made that particular night so bright? Or are nights outside cities normally bright regardless of the altitude (assuming clear sky and near-full moon)?

One thing I suppose could be the reason is that the thinner air due to the altitude meant there was less scattering of the light. I'm not sure if the air was thin enough to make such a difference though.

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  • $\begingroup$ I'm glad you want to participate, but I don't think that this is about Earth Science at all. Rather, it's about the way human eyes adjust to light levels.Someone at Psychology and Neuroscience SE might be able to give you an expert answer. $\endgroup$ – Spencer Jun 11 '18 at 16:58
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    $\begingroup$ This I very much doubt. Of course I have no physical measurements to show, but I believe the brightness was way beyond what eyes adjusting could account for. If that was the case then mankind wouldn't have needed torches to light the way when outdoors under clear skys. $\endgroup$ – Iroh Jun 11 '18 at 17:26
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    $\begingroup$ If there's a moon, especially a full or near full one, it will actually be fairly bright. Then if the ground is mostly fairly light-colored rock, or snow, the light will reflect off that. Even a starlit night can be fairly bright, once your eyes adapt. For a really dark night, you need clouds, and to be a long way (100 miles isn't too much) from city lights that can reflect off the cloud base. $\endgroup$ – jamesqf Jun 12 '18 at 6:10
  • $\begingroup$ Re Anyone who lived outside cities will tell me that. I won't tell you that. I grew up on a farm and went cross country skiing by moonlight many times when I was young. I've backpacked at night, far from any city lights, with the only source of light being the Moon. Your eyes need about 20 to 30 minutes to fully adapt to the nighttime lighting conditions, and after that, moonlight is more than enough to see by. $\endgroup$ – David Hammen Jun 13 '18 at 14:30
  • $\begingroup$ it is not more light but in the thin air the contrast is better so it trick your brain to think it must be brighter. $\endgroup$ – trond hansen Jun 13 '18 at 18:32

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