How much of one day can be considered nighttime, on average?
To further clarify, assume we are on the equator, I want to know how long a time, as a percentage, you could consider to be nighttime on Earth, with the points in time separating night and day being within sunrise and sunset.


I personally wouldn't consider that the dividing line between night and day. I consider night to be the period between dusk and dawn rather than sunset and sunrise. One doesn't need to stop playing a ball game on an unlit field just because the Sun has set. There's still another twenty to forty minutes of light from the blue sky. Dusk and dawn last the shortest in the tropics, with civil twilight (e.g., the period when one can continue playing a ball game on an unlit field) generally considered to be the 24 minute period prior to sunrise / after sunset in the tropics. With this, "nighttime" comprises about 47% of the time in the tropics.

The question however asks about the period between sunset and sunrise. Even on an airless planet, "nighttime" by this definition will comprise a bit less than half the time at the equator because sunrise and sunset occur when the top of the Sun appears above / drops below the horizon. Since the Sun's angular diameter is about 32 arc minutes, on average, this alone means that daytime is about two minutes longer than nighttime+twilight at the equator. We don't live on an airless planet. Atmospheric refraction means that when we see the very top of the Sun at the horizon, the Sun is geometrically well below the horizon.

This, coupled with the Sun's angular size, makes daytime last about 12 hours and 6 to 8 minutes at the equator, with very little variation over the course of a year. Taking 12 hours and 7 minutes as the average, daytime represents 50.5% of the time while twilight plus nighttime (or just night, per the question) represents the other 49.5% of the time.