Beech Tree at the Blue Hour --- Harlem Meer

Dusk is such a subtle time

Did you know there are three different twilights?  Here's Wikipedia's descriptions:

Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon (civil dusk).

Nautical twilight is the time when the center of the sun is between 6° and 12° below the horizon. In   general, nautical twilight ends when navigation via the horizon at sea is no longer possible.

Astronomical twilight is the time when the center of the sun is between 12° and 18° below the horizon. From the end of astronomical twilight in the evening to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution) is dark enough for all astronomical observations.