Twilight is defined in three ways: Civil twilight occurs when the sun is six degrees below the horizon. At the end of civil twilight, you’re legally required to turn on your automobile headlights. The next darker form of twilight is nautical twilight, with the sun 12 degrees below the horizon. During nautical twilight, you can see stars and planets, but it is still bright enough for the horizon to be seen. Astronomical twilight occurs when the sun is 18 degrees below the horizon. At the end of astronomical twilight, the sky has reached its maximum darkness. In polar latitudes, nights may pass where one or more of these may not occur.