The navigator uses meridian pairs that are visible after sunset and before dawn at the time of the voyage. Voyages are planned for times of the year when the weather and winds are optimum and also when certain meridian pairs are visible. For example, when sailing to Hawai'i from the South Pacific, the navigator wants to have the Southern Cross crossing the meridian in the night sky for determining when the canoe has reached the latitude of Hawai'i. The Southern Cross was visible crossing the meridian in the predawn sky as Hokule'a returned to Hawai'i in February, 2000. Latitude-Rising and Setting Pairs Pairs of stars that rise or set at the same time can also be clues to latitude.
Pairs rise and set together only at specific latitudes.
As the observer moves north or south of that latitude, one or the other star will begin to rise or set before or after the other star. It is easier to use setting rather than rising pairs of stars to determine latitude, because the observer can watch the pair as it approaches the horizon instead of trying to anticipate their appearance. At a given latitude, only certain stars will pass through the zenith, the imaginary point in the sky directly overhead. The declination of the zenith star is equal to the latitude it is associated with. If the navigator knows the zenith stars of different latitudes, he can tell what latitude he is at by observing what star passes directly overhead at night.