This image presents a remarkable view of the East Atlantic, stretching from the equator and western Africa northward to Iceland and Greenland. Along the terminator, north to south, are Scandinavia, France, Spain, Algeria, Mali, and the Ivory Coast. But this is not a real photo. It’s a composite of several images taken at different times and different wavelengths, along with Google Earth overlays. Read the details here:
https://earthobservatory.nasa.gov/features/LivingEarth
The transition from day to night is shown in exquisite detail, with the lights of major cities (like Paris) in striking contrast to surrounding dark landscape. Most importantly, note how the terminator (dividing line between day and night) is a diffuse region spanning a few hundred miles. This is where dusk is happening as the terminator moves to the west. On the other side of the planet, dawn is happening.
Earth’s rotation exposes different time zones to sunlight over the period of a day, and that’s why the terminator moves. Its diffuse appearance is caused by Earth’s atmosphere, which allows sunlight to be scattered into time zones where the Sun has already set.
Contrast that with the photo below, showing the terminator on the Moon:
You see no diffusion of the terminator in this case, since the Moon has no atmosphere to scatter light. When the Sun sets on the Moon, it’s like turning off a light switch — day to night almost instantly. I say “almost” since there’s about two weeks of time between when the Sun first touches the lunar horizon and when it dips totally below the horizon. During this period the light gradually dims as less of the Sun becomes visible. But once that last bright sliver disappears below the horizon … “click” … lights off.
Interestingly, you can see some peaks on the night side that are high enough to still catch sunlight, but that happens on Earth too. No surprises there. On a round world, it’s a natural consequence of curvature. And, I might point out, proof that the Earth is not “flat.”
Next Week in Sky Lights ⇒ Formation of the Strait of Gibraltar