when the sun goes down

The sun goes down at sunset, which is defined in astronomy as the moment the upper limb of the Sun disappears below the horizon. The actual time of sunset varies by location, date, and time zone. For example, today in London, England, UK, the sunset time is around 8:00 pm local time. Sunset marks the end of the day and the start of twilight until night fully falls when the sun is about 18 degrees below the horizon.