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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.

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Because:

The tide was low on the first pass.
On the second pass a week later, the tide was high at the same time of day, covering some low-lying islands.

The typical tidal period is two tides every 24 hours 50 minutes. So in 7 days the high & low tides move by about 7 x 50 = 350 minutes = 6 hours, which is the approximate time between low and high tides.

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    $\begingroup$ It could also work the other way around, an island with several tall hills getting split into separate islands during high tide would explain the phenomenon with fewer coincidences required. $\endgroup$ – Bass Dec 26 '20 at 22:45
  • $\begingroup$ @Bass yes, but 10 seconds after I posted was another answer mentioning the reverse sense, and I did not want to take what that said. I don't know what you mean by "fewer coincidences". In both answers, some land gets covered at high tide. $\endgroup$ – Weather Vane Dec 27 '20 at 0:05
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It could be that

six of the islands were barely above sea level; when the tide rose, it covered some of these islands. (Or alternatively, the tide fell and 'merged' six of them into other islands!)

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