Where does the sky begin?

The sky begins where the horizon ends.

The sky we see on any horizon is above someone else’s head… both day and night.

Be warned, this is a very sciencey answer. Be prepared…

According to many sources, the sky is anywhere above the ground. The Earth’s atmosphere doesn’t just stop and then BAM space, the atmosphere thins out gradually as your altitude rises.
The color blue has always been there, so you could say that it originated from the sun, but I know that’s not what you’re asking. The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue. As you look closer to the horizon, the sky appears much paler in color.

At night, your side of the planet is facing directly opposite of the sun’s rays, and since light can’t travel through a planet, there are no light rays for the atmosphere to diffuse. So you see space depending on if there are clouds floating around at that time.

IIf you define the bottom of the sky as the horizon there is an easy answer.

In English units, the distance to the horizon in miles is: 1.23 x the square root of the observer’s height. If you are standing on the shore a few feet above sea (say your eyes are 5 feet above the ground and you are on 4 ft of sand then 1.23 x sqrt(9) = 1.23×3 = ~ 4 miles.
Or you can use this calculator.

Every child knows that the sky does not begin at the ground.