The paths of the stars across the sky depend upon the observer’s latitude. At any given latitude, only certain stars will pass directly overhead. Conversely, any given star will only be seen to pass overhead from one particular latitude on earth. For this to happen, the observer’s latitude has to be equal to the “latitude” of the star on the celestial sphere—in other words, its declination.
The New Zealander David Lewis, in his efforts to trace surviving fragments of traditional methods of navigation in the Pacific, encountered a number of references to “the star on top,” “the overhead star,” “the star that points down upon an island,” and the like. Taken together, these suggest the former existence of a Polynesian tradition in which different islands were seen to have distinctive “marker” stars. Most intriguingly, a Tahitian chant recorded in 1818 listed eight “pillars that hold up the heavens.” The declinations of several of these stars, Lewis found, corresponded in c.e. 1000 (since they change slightly over the centuries, owing to precession) to the latitudes of various southern Polynesian islands, from the Line Islands in the north down to New Zealand in the south. The implication is that the overhead stars, marker stars, and pillar stars could have been zenith stars for particular islands.
The extent to which zenith stars were used by Pacific navigators is another question. Zenith stars cannot be used to plot a course, merely to determine the current latitude. Even to do the latter at all successfully, one has to identify the upward direction with reasonable accuracy, which is of course difficult on a swaying canoe. On the other hand, Lewis himself did achieve this reasonably satisfactorily by siting up the mast, and there are various documented references to zenith stars being used for navigation, particularly in Micronesia. However, even with zenith star observations as a back-up, Oceanic navigators would still have needed to use other methods (such as a combination of star compasses and dead reckoning) in order to locate distant islands.
