User: From the passage provided, summarise the key phases of sunset.
Assistant: 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.