There is a cluster of islands, which a private pilot flies over at dawn. The pilot can see 17 islands. A week later, she makes the same flight, at dawn, following the same altitude, speed, path and direction, and this time she sees 11 islands. Why did the count change?
The pilot has perfect vision, a clear view of the entire area, and she makes no errors or omissions. The islands are natural, solid bare rock, static (not erupting, uplifting, subsiding, etc), and not subject to any human engineering.