I read on HN that they found islands over the horizon by looking at patterns in the waves and navigated by memorizing the position of stars. Yes to the harvesting of rain water.
By wave patterns (the island would reflect waves, and they could detect that in a canoe), by birds (land-based birds fly back to the island in the evening), and by clouds (certain cloud formations build up over land). Combine them all, and they could know where islands were that they couldn't see. The reflected wave thing worked even at night.
> Combine them all, and they could know where islands were that they couldn't see.
I'm curious as to the range that these things could be detected. No doubt there's some proportionality between island size and distance-detectability.
But there are a whole bunch (small) islands in the middle of a whole lot of nothing, so I'm curious to know how many were found on purpose and how many by accident.
The Hawaiian islands in particular are so large that they affect wind and cloud patterns for hundreds, sometimes thousands of miles away. You can see this happening in this satellite photo: https://upload.wikimedia.org/wikipedia/commons/6/6b/Hawaje-N... Look at how the clouds and currents change from the top right down to the bottom left.
