ASTRONAUTS ABOARD the International Space Station experience, on average, 16 sunrises and sunsets during each 24-hour orbital period (as an orbit takes only approximately 90 minutes).
Each changeover between day and night is marked by the ‘terminator’, the dividing line on Earth’s surface separating the sunlit side from the darkness.
While the terminator is often conceptualised as a hard boundary—and is frequently presented as such in graphics and visualisations—in reality the edge of light and dark is blurred due to the scattering of light by the Earth’s atmosphere.
On the ground, we experience this zone of diffuse lighting as dusk or twilight—although the Sun is no longer visible, some illumination is still present due to light scattering over the local horizon.
The terminator is visible in this panoramic view across central South America, looking towards the northeast. An astronaut took the photo at approximately 7:37pm local time.
Layers of the Earth’s atmosphere, coloured from bright white to deep blue, are visible on the horizon (or limb). The highest cloud tops have a reddish glow as they pick up 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.
Astronaut photograph provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Centre. Text adapted from information issued by William L. Stefanov, NASA-JSC.
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