The Latitude Hook

The latitude hook was a crude but very useful navigational tool developed by the inhabitants of the Pacific Islands. It is essentially a stick with a loop at one end.

It works on the principle that we perceive stars to rotate around the celestial poles. Polaris, the North Star, is approximately directly above the North Celestial Pole, and for observers in the Northern Hemisphere, the altitude of Polaris above the horizon actually gives an approximation to the observer's latitude. This is exactly what the latitude hook does. It gives readings of the angle between the horizon and Polaris.

So for a navigator in the Pacific Islands above the equator, when the stars are visible, he would align his latitude hook at a fixed distance from himself, say one arm's length, such that Polaris were visible when he looked into the loop. As he continued out at sea, he could have gone northwards to a higher latitude or southwards to a lower latitude. If he had gone north, then maintaining the same one arm's length distance of the latitude hook from himself, Polaris would appear to be above the loop. To get back to his original latitude, he would simply head southwards till Polaris appeared in the loop again.

North Celestial Pole

For natives living in the southern Pacific Islands, things were not quite the same. The south celestial pole does not have any marker star such as Polaris for the north celestial pole. In order to locate the south celestial pole, the native navigators plotted two imaginary lines. The first line passed through the stars Gamma Crucis and Acrux in the Southern Cross constellation, and the other line is the perpendicular bisector of the line joining the two stars Agena and Rigel Centaurus which are in the constellation Centaurus. The intersection of these two lines gives a rough position of the south celestial pole.

North Celestial Pole

Of course, native navigators in the south islands had a harder time owing to the fact that their latitude hooks showed only an imaginary intersection point which was not actually visible through the ring.

The latitude hook is susceptible to two main errors which affect the accuracy of observations. The first is that of parallax, which occurs when a person misjudges an object's actual positioning. The second error arises when the distance of the latitude hook from the observer is altered. This could happen quite easily by simple shoulder movements, if the distance is taken to be one arm's length.

Though it was crude and only gave an approximation, the latitude hook was easily constructed and extremely useful to the natives at their time.

1. Dennis Fisher Latitude Hooks and Azimuth Rings 1995, International Marine