coconut ring

Navigation must have been “by guess and by God,” as the saying goes.
One old schooner skipper asked Klebingat if he had a spare Nautical Almanac. “Now I’ll come fifteen or twenty miles closer to my destination,” he happily told Klebingat on receiving the precious book.
As for the natives in their big outrigger canoes, east or west was guesswork. But latitude was more exact.
Their “sextant” was half a coconut bored with a ring of holes the right distance below the rim. One sighted through a hole, across the far rim at a fixed star. The instrument was leveled by half filling it with water and keeping its surface aligned with a line circling the inside of the shell.
—  Cruise of the Lanikai, by Kemp Tolley