Civil twilight, nautical twilight and astronomical twilight
Civil twilight is the time before sunrise (and after sunset) when the sun is still not more
than 6 degrees below the horizon.
At this time it is still possible to carry out most of the common
activities without artificial lighting.
Nautical twilight is the time preceding or following civil twilight - the sun is between 6 and
12 degrees below the horizon. You can already see
the brightest stars and sea horizon, that was previously used to
navigate (that is how originated the name of this twilight).
Astronomical twilight is the time preceding or following nautical twilight - the sun is between 12
and 18 degrees below the horizon.
At this time, the sky is still faintly illuminated by sunlight
dispersed in the atmosphere, so you can not see the least bright stars.