From the passage provided, summarise the key phases of sunset.
Sunset is divided into three stages. The first is twilight, which begins once the Sun has disappears 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. The third phase is astronomical twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon.