How does one without modern aids estimate the distance travelled and his position during a voyage? The Micronesian navigators have an elegant and ingenious method to do that. They call it Etak. It involves using, as reference, an island off to one side of the course. To the navigator, the island actually moves under the star points on the horizon behind it!
This island, called lu pongank, simply means "in the middle and athwart". It is not actually seen during the voyage as the real island might be far away. Some might just be ghost islands, imaginary points of reference for the Etak process, since there are no conveniently located islands (like for the trip north from the Carolines to Guam and Saipan).
To illustrate the whole process, let's take the example of sailing from Satawal to West Fayu, taught by Uurupa to the author in The Last Navigator. The reference island here is Lamotrek.
From wofanu of Satawal the navigator knows that West Fayu lies under the setting position of the Little Dipper and Lamotrek under setting Tarazad. From wofanu of West Fayu, Lamotrek lies under setting Shaula. Between setting Tarazad and setting Shaula lies five star points. They are setting Altair, setting Alshain, setting Orion's Belt, setting Corvus and setting Antares. As he sails, he can mentally track his position along the course by visualizing the reference island moving along the horizon, under the stars. If he sees the island under the next star point, he knows he has reached the next leg of his journey.
Journey from Satawal to West Fayu
In a journey, there are also two points that give clue about his position. They are the etak of sighting and the etak of birds. The etak of sighting is the point at sea at which the island slip beneath the horizon. On all voyages there are two etak of sighting, one for the departure island and one for the destination island. The etak of birds is the second (and second to the last) etak of the voyage. It is simply the furthest range of certain seabirds which at dawn fly 18 to 20 miles from their islands to fish for the day and at dusk return to their home. These birds fly a direct course to and from their islands. Thus, if the navigator is unsure of where his destination island lies but is inside the etak of birds, he can wait for dawn or dusk and observe carefully the flight paths of the birds.
Diagram for Satawal-West Fayu journey
1 - Setting Tarazad
2 - Setting Altair
3 - Setting Alshain
4 - Setting Orion's Belt
5 - Setting Corvus
6 - Setting Antares
Thomas, Stephen D. (1987). The Last Navigator.