astronomical twilight
Technical term in astronomy for the period when the sun is between 12 and 18 degrees below the horizon; the sky is dark enough for most astronomical observations.
When astronomical twilight ends, the sky becomes completely dark.
Civil twilight: sun is 0–6 degrees below the horizon; brighter than astronomical twilight.
Nautical twilight: sun is 6–12 degrees below the horizon; intermediate brightness.
Compound of 天文 (astronomy) and 薄明 (twilight).