天てん文もん薄はく明めい

noun

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.

• 天てん文もん薄はく明めいが終おわると、空そらは完かん全ぜんに暗くらくなる。

  When astronomical twilight ends, the sky becomes completely dark.