Source: Futility Closet
Navigators from the Poluwat atoll of Micronesia find their way among islands by appealing to parallax: A reference island at one side of their course will appear to pass beneath a succession of stars:
The star bearings of the reference island from both the starting and ending points of the trip are known, since on another occasion the reference island may itself become a destination.
In between there are other navigation star positions under which the reference island will pass as it ‘moves’ backwards. Its passage under each of these stars marks the end of one etak and the beginning of another. Thus the number of star positions which lie between the bearing of the reference…