Definition
The period of incomplete darkness before sunrise and after sunset, during which the sun is below the horizon but its light is still scattered through the upper atmosphere. Aviation recognizes three defined stages based on the sun's angle below the horizon: civil twilight (sun 0° to 6° below), nautical twilight (6° to 12° below), and astronomical twilight (12° to 18° below).
Plain English
The dim part of the day after the sun has set or before it has risen, when there is still some natural light in the sky even though the sun itself isn't visible.
Context Anchor
Pilots encounter twilight in flight planning, takeoff and landing decisions, airport lighting use, and rules that depend on whether an operation is considered day or night.
Derivation
From Old English 'twi-' (two, or half) and 'light' — literally 'half light' or 'between light.' The 'two' sense reflects the in-between state: not full day, not full night.
Why Pilots Care
Determines when night flight rules apply, when pilots may log night time, and when specific VFR weather minimums or currency requirements become effective.
Grounding Statement
Just after sunset, you may still be able to see the runway clearly, but the outside light is already fading and changing quickly.
Intuition Check
Twilight does not simply mean “night.” It is the transition period between day and night, and aviation rules may treat parts of that period differently depending on the operation.
Example Sentence 1
He landed during evening civil twilight, so his position lights had to be on, but the flight didn't count toward his night currency.
Example Sentence 2
Flight planning software showed civil twilight ended at 1842 local, after which night VFR minimums applied.