Eli5; Where does the sky begin?

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Does the sky start above the trees? Or does the sky start on the ground?

In: Planetary Science

2 Answers

Anonymous 0 Comments

the sky starts at your eye. “The sky” is just a 100km of air. Air is ever so slightly blue, but not very blue, and you only see it when bright light is shining through about 20km of air (like the sun in the daytime).

But the reason the sky is blue is the same reason distant objects appear blue. Air is ever so slightly blue. It doesnt really “start” anywhere in particular, it just kinda “Fades in” relative to where you are looking from

Anonymous 0 Comments

Excellent question – truly one that would be asked by a 5-year old, and hence, merits an answer that a 5-year old would understand.

Look at the ocean. It looks blue, right? But scoop up a few droplets. Do those look blue? So.. where does the blue-ness begin? Can you sail out to a point where the blue-ness starts?

The answer? Both the blue-ness of the ocean and the sky are emergent properties of our angle, the sun’s light composition, and the angle of the sun relative to us and the sky/ocean. Change it and it will change the phenomenon as we know it. There is no “start” per se. If you hop on a rocket and shoot up, the sky will gradually fade from its bright blue to darker and darker tones until it becomes black. You don’t ever punch through a wall of blue.

So, where does the sky begin? Maybe it’s better to ask if the sky even exists, or if it is just an optical illusion.