January 5, 2015
There are three classifications of 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.” 
It starts as soon as the sun dips below the western horizon. 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.
‘Several countries use this definition of civil twilight and civil dawn to make laws related to aviation, hunting, and the usage of headlights and street lamps.
In the United Kingdom, the time when everyone has to switch on their headlights is known as hours of darkness, which is 30 minutes before sunrise and after sunset.’ )
It begins when it’s fairly dark outside. By definition, nautical twilight 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.
‘The term, nautical twilight, dates back to the time when sailors used the stars to navigate the seas. During this time, observers on Earth can easily see most stars.’ )
It ends when all traces of sky glow are gone. By definition, astronomical twilight ends when the sun is 18 degrees below the horizon. Then astronomers can begin to observe the stars, assuming no clouds are in the way!” 
‘It is that point in time at which the sun starts lightening the sky. Prior to this time during the morning, the sky is completely dark. During the evening, this is the point where the sky completely turns dark.’