Astronauts onboard the International Space Station see, on average,
16 sunrises and sunsets during a 24-hour orbital period. 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—and is frequently presented as such in graphics and visualizations—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.
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.
Astronaut photograph ISS027-E-12224
was acquired on April 12, 2011, with a Nikon D3S digital camera using a
50 mm lens, and is provided by the ISS Crew Earth Observations
experiment and Image Science & Analysis Laboratory, Johnson Space
Center. The image was taken by the Expedition 27 crew. The image has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab
to help astronauts take pictures of Earth that will be of the greatest
value to scientists and the public, and to make those images freely
available on the Internet. Additional images taken by astronauts and
cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by William L. Stefanov, NASA-JSC.