Pick any place, date and time. Then see whether you could read a sign from where you stand.
Choose any place on Earth, a date, a time and a distance. The page works out how much light there is at that moment and shows you a white road sign as your eyes would see it.
The sun and moon positions come from the place and time. They give the light on the ground: about 400 to 600 lux at sunset, about 3 lux when civil twilight ends, and about 0.008 lux when nautical twilight ends. Moonlight, starlight and light from nearby towns are added on top.
Yes, and that is why overcast is the default. Away from towns, cloud blocks much of the twilight, so you lose the sign earlier. In Oslo on 1 November, 20 cm letters at 30 metres stop being readable about 16 minutes earlier under overcast than under a clear sky. Near cities at night it reverses, because clouds reflect city light back down.
The eye sees less detail as light fades. The page uses published measurements of eyesight at different light levels, and counts text as readable when the letters are at least twice the smallest size you can just make out.
Treat the times as good to about 15 to 20 minutes. Real cloud varies a lot, and the model leaves out street lamps, snow on the ground and local terrain. Screens cannot show real starlight, so go by the readable or not readable result on very dark nights.
How Dark Is It needs JavaScript and WebGL 2 to show how dark it is at any place, date and time.