How Dark Is the Night Sky?
Briefly

"During the day, sunlight entering the atmosphere is scattered by molecules such as nitrogen and oxygen at more or less random angles, making the sky appear bright and blue."
"Even at night, the sky isn’t completely dark; there is still some light, from sources like the moon, city lights, and a faint nocturnal glow from atmospheric molecules."
Read at www.scientificamerican.com
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