Astronauts onboard the International Space Station see the Earth from a unique perspective -- for example, in one 24-hour period, they see not one sunrise and sunset, but 16 on average.
Each changeover between day and night is marked by the terminator, a line on Earth's surface separating the sunlit side from the darkness.
While the terminator is often conceptualized as a hard boundary, in reality the edge of light and dark is diffuse due to the scattering of light by the Earth's atmosphere . This zone of diffuse lighting is experienced as dusk or twilight on the ground; while the Sun is no longer visible, some illumination is still present due to light scattering over the local horizon.
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The terminator is visible in this panoramic view across central South America, looking towards the northeast. An astronaut shot the photo at approximately 7:37 p.m. local time.
Layers of the Earth's atmosphere , colored bright white to deep blue, are visible on the horizon (or limb). The highest cloud tops have a reddish glow due to direct light from the setting sun, while lower clouds are in twilight.
The Salar de Coipasa, a large salt lake in Bolivia, is dimly visible on the night side of the terminator. The salar provides a geographic reference point for determining the location and viewing orientation of the image.
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