Donald Gansauer of Baltimore asks: "What defines twilight, and why are there different times for it: civil, nautical and astronomical?" Evening civil twilight (used in headlight and burglary laws) extends from sunset until the sun's center is 6 degrees below the horizon. Nautical twilight (when navigators can still see stars and the horizon) ends when the sun is 12 degrees below. Astronomical twilight is until the sun is 18 degrees below, when the dimmest naked-eye stars become visible. Reverse for morning twilight.