"We catch a glimpse of a huge swirl of clouds out the window over the middle of the Pacific Ocean, or the boot of Italy jutting down into the Mediterranean, or the brilliant blue coral reefs of the Caribbean strutting their beauty before the stars. And...we experienced those uniquely human qualities: awe, curiosity, wonder, joy, amazement." (Russell L. Schweickart, Apollo Astronaut ("The Home Planet")
Space Station Orbit Tutorial
Cynthia A. Evans and Julie A. Robinson, Earth Sciences and Image Analysis, NASA Johnson Space Center
FACT 4: The part of the Earth visible to ISS astronauts in daylight changes due to the interaction between the orbit
patterns of the station and the rotation of the Earth. The daylight portion of the orbits shifts slightly eastward along
track each day. This lighting procession follows an approximate 63-day cycle from a descending track covering the mid-latitudes,
to southern hemisphere lighting, to ascending tracks, to northern hemisphere lighting. This cycle, plus seasonal changes in
solar incidence, cause the sun illumination angles to vary every time the station passes over a given region.
Figure 5. The cycle of daylight procession, from top to bottom: A) 1 day of orbits with daylight on descending pass.
Figure 6. B) 1 day of orbits with daylight in S. Hemisphere.
Figure 7. C) 1 day of orbits with daylight on ascending pass.
Figure 8. D) 1 day of orbits with daylight in N. Hemisphere.