I had watched the earlier 9:04 p.m. crossing of the ISS and this lasted nearly 5-minutes at almost the same elevation as the 10:39 p.m. crossing.  So why was the 9:04 p.m. crossing 5-minutes long and the 10:39 p.m. crossing only 3-minutes long?  The accompanying image tells the story.  At 10:39 p.m. the Sun has set further, so the ISS is illuminated for less of its course across the sky.  As you can see in the image – it barely makes Ursa Major before its lights out – fascinating!

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