What is the definition of astronomical twilight?
Quick Answer: Astronomical twilight is the time when the Sun is between 12° and 18° below the horizon. During this period, the sky is dark enough for astronomers to observe stars and other celestial objects without interference from sunlight.
Astronomical twilight happens in the early morning before sunrise and in the evening after sunset. It marks the transition between night and complete darkness.
When the Sun is more than 18° below the horizon, the sky is fully dark. Between 12° and 18°, the Sun’s light is still faintly scattered in the atmosphere, but it doesn't brighten the sky enough to affect serious astronomical observations.