October 26, 2014

Latitude hooks and daring feats

I’M NOT SURPRISED that ancient Polynesian sailors managed to find their way from remote Easter Island to the mainland of South America. What interests me far more is how they found their way back. Easter Island is, after all, a tiny speck of land 2,300 miles west of South America, and 1,100 miles away from any other island.

Scientists recently conducted a study that shows interbreeding between the native peoples of Easter Island and those of South America. They believe it occurred between 1300 and 1500, and the genetic evidence shows that it was probably the Rapa Nui people of Easter Island who made the long ocean voyages there and back.

The Polynesians had fast, seaworthy sailing canoes, it’s true, but can you imagine how skilled they were as navigators at a time when most European sailors were clinging cautiously to the coastlines in slow, dumpy vessels that were little better, if any, than those of the ancient Greeks and Romans?

How could they hope to make an accurate landfall on Easter Island after the long lonely trek from South America? It is nothing less than astounding.

I know that one of their tricks was to place themselves on the latitude of an island and then to run either due east or due west until they hit land. And one of the devices they used to determine their latitude was a straight piece of split bamboo with a loop at the top. This straight piece had a shorter piece of bamboo, known as the pointer, tied at right angles to the looped piece.

I learned this some years back when I edited Dennis Fisher’s wonderful little book called Latitude Hooks and Azimuth Rings (International Marine). The latitude hook relies on the fact that the stars appear to rotate around fixed points known as the celestial poles. In the northern hemisphere, for instance, the north celestial pole is marked, as near as dammit, by Polaris, the North Star. With a latitude hook held at arm’s length, and the pointer aligned with the horizon, a navigator knew he was maintaining his correct latitude if he could see Polaris through the loop on the top.

If the star were above the loop, he would head farther south; if below the loop, he’d head north, In the South Pacific, he’d use the constellations Southern Cross and Centaurus to figure out the position of the south celestial pole.

While it sounds very simple, it must have taken some skill to use a latitude hook with any degree of accuracy, and I suspect that the final landfall was achieved with the aid of other navigational tricks learned by the Polynesians, such as their ability to deduce the position of an island still hidden over the horizon from the angle of reflected swells.

However they did it, it was a marvelous achievement for the times, and if you’d like to try your hand at it some time, get hold of a copy of Fisher’s book, because it also tells you how to build and use 18 traditional navigational tools, including the astrolabe, the cross-staff and the octant. Even if you don’t build any of the instruments, it’s a fascinating read.

Today’s Thought
Navigare necesse est; vivere non est necesse.
— Gnaeus Pompeius Magnus

“Paddy, you should be more careful about pulling your drapes at home. When I drove past your house last night I distinctly saw you kissing your wife.”
“Ha, well, then the joke’s on you, O’Riordan. I wasn’t home last night.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

No comments: