What term describes the boundary between day and night on the Moon's surface?

• A. Libration
• B. Rille
• C. Craters
• D. Terminator

Answer: (D)

The boundary between day and night on the Moon’s surface is called the terminator. This line marks where sunlight just reaches the surface, so one side is illuminated while the other is in darkness. On the Moon, nearly no atmosphere means there’s little atmospheric scattering, so the terminator appears as a relatively sharp boundary rather than a gradual twilight. As the Moon rotates, the terminator sweeps across the surface, and the lighting along it creates long shadows that help reveal craters, mountains, and other features. Other terms refer to different ideas: libration is the slight wobble that lets us see a bit more than half of the Moon over time; rilles are long channel-like valleys; craters are impact basins. Therefore, the term that best describes the day-night boundary is terminator.