How light does it have to be to be considered dark?

Question

I was told at a young age that anything is possible. So, how light does it have to be to be considered dark?

Answer 1

When the light is rapidly fading and the sun is about to go completely down.

Answer 2

Civil night starts when the Sun is 6 degrees below the horizon.

Nautical night starts when the Sun is 12 degrees below the horizon.

Astronomical night starts when the Sun is 18 degrees below the horizon.

Answer 3

Your question could also be: How dark....to be considered light?

There's a problem in Western thought dating back to Plato (or earlier) about our  logical or verbal categories, such as 'light' and 'dark' and the realities of nature, which is full of continua. The intensity of light varies continuously; for any difference one can describe a smaller one. The variation is smooth. The question then is how to cut the continuum up into sections, and where to draw their boundaries becomes an aribtrary decision, meaning the limits might just as well be put in one place as another.

So our categories don't match the reality; they may be handy in idle conversation, but they actually convey little information. All such questions are thus more than slightly silly.

Finally, what you were told as a kid is wrong; not everything is possible.