Definition
The period before sunrise and after sunset during which the center of the sun is between the horizon and 6 degrees below the horizon. During civil twilight, there is enough natural light to see terrestrial objects clearly without artificial illumination. The end of evening civil twilight and the beginning of morning civil twilight are used in U.S. aviation regulations to define the boundaries of night for purposes such as position-light requirements and night-currency rules.
Plain English
The window of dim natural light just before sunrise or just after sunset, when the sun is below the horizon but still close enough to light the sky. In flying, this window is used to mark when 'night' officially begins and ends.
Context Anchor
Pilots encounter civil twilight when determining official night time for certain FAA rules, logbook entries, and flight planning.
Derivation
From Latin 'civilis' (relating to citizens or ordinary life) and 'twilight' (the half-light between day and night). 'Civil' here means the level of light considered adequate for ordinary outdoor activity — not a technical or astronomical level. This helps explain why it sits between full daylight and the darker definitions of twilight used by astronomers.
Why Pilots Care
Marks the exact point when night flight time begins for pilot logbook entries under 14 CFR 61.51.
Grounding Statement
If the sun has just set but the sky is still light enough to see the ground clearly, you are likely in evening civil twilight.
Intuition Check
Civil twilight does not mean any time the sky looks partly dark. In aviation, it is an official time period based on the sun being within 6 degrees below the horizon.
Example Sentence 1
The pilot turned on the aircraft's position lights at the end of evening civil twilight, as required by regulation.
Example Sentence 2
During civil twilight the runway remained visible without aircraft lights.