Sunset is distinct from twilight, which is divided into three stages, the first being "civil" twilight, which begins once the Sun has disappeared below the horizon, and continues until it descends to 6 degrees below the horizon; the second phase is "nautical" twilight, between 6 and 12 degrees below the horizon; and the third is "astronomical" twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon. "Dusk" is at the very end of astronomical twilight, and is the darkest moment of twilight just before night. Night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky.