This picture was taken from the International Space Station orbiting at an altitude of 390 km. The terminator is the line between day and night. In pictures of airless moons it’s a firm line, but no such sharp boundary marks the division between day and night in this picture of ocean and clouds on Earth. Instead, the shadow line is diffuse and shows the gradual transition to darkness as twilight falls. The Sun illuminates the scene from the right, and the cloud tops reflect gently reddened sunlight filtered through the troposphere, the lowest layer of the atmosphere. The upper atmosphere scatters blue sunlight and fades into the blackness of space.
Image Credit: NASA