What is twilight?
Photo: Flickr user aqui-ali
Why do we have twilight?
We have twilight because Earth has an atmosphere. Some light scatters through small particles in the atmosphere – so there’s still some light in the sky even after the sun has gone down. You can define twilight simply as the time of day between daylight and darkness, whether that’s after sunset, or before sunrise. But this time of day is important to people for a lot of reasons, so some more strict definitions have evolved on the subject of twilight.
First, there’s civil twilight. It starts just after the sun goes down. There’s enough light to see – but people turn on their lights to drive a car – and the streetlights are starting to come on. Civil twilight officially ends when the sun is 6 degrees below the horizon. Then it’s fairly dark outside – what’s called nautical twilight, which ends when a distant line of a sea horizon stops being visible against the background of the sky – about when the sun is 12 degrees below the horizon. And even then some people still call it twilight. Astronomical twilight ends when all traces of sky glow are gone – when the sun is 18 degrees below the horizon.
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