I saw an excellent documentary about an optical illusion that occurs in the North Atlantic, when the weather is just right. There have been photos taken that seem to show boats floating in the air. The weather was right for that illusion on the night of the sinking. So, when the lookouts saw the iceberg, it would have appeared much higher, on the horizon, instead of where it actually was. That made their calculation of distance wrong. They'd have expected to have plenty of time to turn and miss it. Until the illusion collapsed, and suddenly the iceberg was Right There.