astronomical twilight

American  

noun

1. either of two periods of partial darkness, after sunset or before sunrise, when the center of the sun is more than 6° but less than 18° below the horizon.

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Example Sentences

As with anywhere else on Earth, the middle of the day can be lighter than other times as the Sun ascends toward and falls away from the horizon without ever rising above it: “civil twilight” may provide enough light for people to carry on with their outdoor activities, while “astronomical twilight” is so dark that the only natural lights are stars, the Moon, and the aurora borealis that dance and flicker across a clear sky.

From National Geographic

In the morning in Seattle on June 27, astronomical twilight, which is when the sun is 18 degrees below the horizon, began at 2:07 a.m.

From Seattle Times

In theory, at least, the last lingering vestiges of the sun’s illumination do not give way to total nighttime blackness until the end of what is called astronomical twilight.

From Washington Post

It is not guaranteed that anyone’s eyesight will be so sensitive as to actually note the precise start of Saturday’s or any morning’s astronomical twilight.

From Washington Post