Definition
The period before sunrise or after sunset during which the center of the sun is between 12 and 18 degrees below the horizon. During astronomical twilight, the sky is dark enough for most astronomical observations, and to the unaided eye it generally appears to be full night.
Plain English
A formal stage of twilight that happens just before dawn and just after dusk, when the sun is far enough below the horizon that the sky looks essentially dark, but a tiny amount of sunlight is still scattered high in the atmosphere.
Context Anchor
Seen in daylight, night, weather, and planning references that describe how much natural light will be available before sunrise or after sunset.
Derivation
Astronomical comes from the Greek astron, meaning star, combined with nomos, meaning law or arrangement. The label fits because this stage of twilight is defined by what astronomers can see: once the sun drops more than 18 degrees below the horizon, the sky is dark enough for serious star observation.
Why Pilots Care
Marks the point at which night flight time may be logged and certain lighting requirements begin to apply.
Grounding Statement
During astronomical twilight, the world may still look mostly dark even though the Sun is beginning to affect the sky slightly.
Intuition Check
Do not assume astronomical twilight is the same as ordinary sunset, sunrise, or legal night. It is a specific low-light period based on the Sun being 12 to 18 degrees below the horizon.
Example Sentence 1
By the time astronomical twilight ended, the airfield was in full darkness and only the runway lights were visible.
Example Sentence 2
During astronomical twilight the horizon remained visible enough for visual navigation.