Twilight is the period after sunset and before sunrise. It has three types: civil, nautical, and astronomical. Civil twilight starts at 6 degrees below the horizon and allows the visibility of the brightest stars and planets. Nautical twilight is at 12 degrees and allows sailors to take readings. Astronomical twilight occurs at 18 degrees, during which only moderately faint celestial objects can be seen.,urban%20or%20suburban%20light%20pollution.