How is the daytime period defined for air traffic control time measurements?

• A. The time from sunrise to sunset
• B. The time from morning civil twilight to evening civil twilight
• C. The time from the beginning of morning civil twilight until the end of evening civil twilight
• D. The time from noon to midnight

Answer: (C)

Daytime for air traffic control time measurements is defined by civil daylight boundaries, not just sunrise and sunset. The period starts at the beginning of morning civil twilight and ends at the end of evening civil twilight. Civil twilight is the window around sunrise and sunset when there is enough natural light for outdoor aviation operations, defined by the sun being about 6 degrees below the horizon. Using this window provides a consistent, usable daylight period across seasons and locations, which is why it’s preferred over simply sunrise-to-sunset. The other options don't align with this standardized daylight window: sunrise-to-sunset ignores twilight before morning and after evening; morning civil twilight to evening civil twilight would end too early, before full twilight ends; noon-to-midnight is unrelated to actual daylight boundaries.