On one train ride north, Wahram listened to one of the reef engineers explain that the corals they were replanting all released their eggs on the same night of the year, and even within the same twenty minutes, though they were scattered over hundreds of miles. Apparently they accomplished this by way of two color-sensitive cells in each coral, which together were able to distinguish the particular blue of the twilight sky on the night after the first full moon following the spring equinox. This moon rose right after sunset, when the sky was still also lit by the recently departed sun, and
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