1) Astronomical phenomenon 2) Evening event 3) Horizon happening 4) Horizon occurrence 5) Horizon sight 6) Lunar disappearance 7) Nautical almanac listing 8) Nighttime view 9) Regular lunar event 10) When the night darkens 
• (n.) The descent of the moon below the horizon; also, the time when the moon sets.
For technical definition, please follow this link to the U.S. Naval Observatory Astronomical Applications website. 
Moon'set` noun The descent of the moon below the horizon; also, the time when the moon sets. 
the setting of the moon below the horizon. · the time at which the moon disappears below the horizon.