When does night exist:
a) During the hours of darkness
b) From 30 minutes after sunset until 30 minutes after sunrise
c) from the beginning of evening civil twilight until the beginning of morning civil twilight.
d) During the period when the centre of the sun's disc is 6 degrees below the horizon
For the purposes of logging hours here in the US, night time is logged 60 minutes after sunset until 60 before sunrise ( Similar to option B ). However, Night is defined by C..