Astronomical twilight is the last of the light, with the sun 12° to 18° below the horizon; when it ends the sky is as dark as it will get.

From 12° to 18° below, the sky is dark to the eye but not to a telescope: the faintest stars and the Milky Way wait for its end. Near midsummer, anywhere north of about 48°, it never ends and the sky never gets fully dark — Paris, Vancouver and everywhere beyond keep a glow in the north all night.

Below 18° the sun no longer lights any of the air above you; what glow remains is the stars, the moon, and the city.