For thousands of years, the people of the Marshall Islands of Micronesia have found their way around a widely dispersed group of low-lying islands, sailing seemingly effortlessly from one atoll to another far beyond the horizon. They had no maps or magnetic compasses, no clocks, no weather forecasts and certainly no GPS or SatNav equipment.
Traditionally, fishing was the main source of food for the islanders, so being able to get to the fishing grounds and back home without too much effort was vital. Sailors who knew seafaring had a high status because the islanders depended on them for their food supply.
Navigators used many methods to find their way. They could navigate by the stars, they knew the habits of seabirds, they understood the clouds, and they were familiar with the ways of the winds. But most important among the tools of their trade was an uncanny ability to understand the motions of the ocean, to read the many wave trains that pattern the surface of the water, giving them vital clues about distant landmasses.
Outrigger canoes react to water and sailors can “feel” the waves and swell due to the movement of the craft. They say wave riding is not just about visual cues but also involves the stomach.
A one-dimensional signal, like a sound sequence, can be analyzed into simple wave-like components, each with a different period. This is called spectral analysis. Similarly, patterns in two or more dimensions, such as waves on the surface of the ocean, can be spectrally partitioned, but now the components have different directions and periods. French mathematician Joseph Fourier was the first to show how a general waveform can be separated into simple components.
Ocean waves are separated into sea and swell. Sea waves are locally generated by the direct action of winds and tend to be steep, slow, transient and chaotic. Swell is the long, regular and rapid undulating movement generated by distant storms and often traveling thousands of kilometers.
The swell is persistent and often remains unchanged for several days. Swell waves have long periods compared to sea waves and are easily distinguished from them by an experienced sailor. The swell provides a stable reference direction, allowing an islander to maintain a straight course to a distant destination.
The northeast trade winds that blow persistently near the islands generate their own trade wind swell, with a shorter period than the swells coming from further away. This provides another benchmark as the direction of trades tends to be quite persistent.
When a regular train of waves encounters an island, it is distorted, producing complex patterns of reflected and refracted waves. Islanders can visually analyze these patterns, essentially performing spectral analysis in their minds. It’s as if they could do a Fourier mental analysis.
This allows them to assess the location of the island, even if there is no visible evidence on the horizon. As they get closer to land, more complex shadowing and diffraction patterns help them find openings through the reefs to the safety of the lagoon.
Scientists worked with the islanders to better understand their extraordinary navigational skills. Computer models can reproduce many of the effects they use, but there is still much to learn. And time is running out: even a small rise in sea level can have dramatic consequences for these low-lying islands.
Peter Lynch is Professor Emeritus at UCD’s School of Mathematics and Statistics – he blogs at thatsmaths.com