Image: Sunset Over Western South America As Seen From Orbit

"Sunset over western South America is featured in this image photographed by an Expedition 27 crew member on the International Space Station. Crew members onboard the space station see, on average, sixteen sunrises and sunsets during a 24-hour orbital period. Each changeover between day and night on the ground is marked by the terminator, or line separating the sunlit side of Earth from the side in darkness. While the terminator is conceptualized as a hard boundary - and is frequently presented as such in graphics and visualizations - in reality the boundary between light and dark is diffuse due to scattering of light by Earth's atmosphere."