Skip to main content


Presented by Mark Roye, PNW station


For centuries the great navigational problem  in repeatedly returning to a destination, whether on the outbound or the return voyage, was in replicating the latitude of both ports. We know that for centuries mariners employed “latitude sailing” or “latitude keeping” to reach intended destinations.  By “running their easting down” or the reverse when westbound, and keeping close to the known latitude, they could make landfall relatively close to the intended destination, then employ known landmarks and primitive coastal piloting to come to port.


Meridian altitude and declination of the sun was was fairly well understood and had been fully described by the early 8th century. Primitive measurements of declination could be accomplished by numerous devices fairly early on as well.


We know that Arab merchant-sailors were very successful not only in the Mediterranean, but other seas as well very early on.


It is also known that Arab ships employed “latitude keeping” as a way to return to a port from which they had departed or which they had previously visited. Therefore they had to have used some device to let them keep to a desired latitude.


Indeed they did have such a device.


It was known as the “Al-Kemal”, a very simple device to record, although not quantify, latitude. The first known description to the western world was by Andrea Bianco in his map of the world in 1436, then later it was taught at Prince Henry the Navigator's school at Sagres. By then it had already been in use for centuries in the Arab world.


The device consists of a simple flat wooden card, with a small notch cut into the top edge at the center. In the middle of the card is bored a small-diameter hole through which a length of twine is secured by knots on each side.


It works like this. The user finds Polaris and settles it in the notch at the top, then extends or retracts his arm, the wooden card in his hand, until the bottom of the card aligns with the still visible horizon. Then, with his other hand, the string is pulled to the tip of the nose, held between two fingers, and then a knot is tied at that location.  A note is made of which knot corresponded to which port or location the mariner has visited and to which he intends to return. By this simple means the latitude has not been quantified, but it has been recorded. The principle is the same as the technique employed by a hockey, lacrosse, or soccer goalie to prevent a successful shot-on-goal by the opponent. By coming out from the goal toward the attacking player, the angle of the goal to the attacker is changed. We know the phenomenon as parallax, defined as, the effect whereby the position or direction of an object appears to differ when viewed from different positions, erg. through the viewfinder and the lens of a camera.”


The distance of the wooden card from the observer's nose, as recorded by the knot in the twine, uses parallax to define the angle of Polaris (viewed in the notch) to the horizon (viewed at the bottom of the card). By keeping or returning to the latitude at which this angle replicates that recorded by the knot of the device it is possible to navigate by “latitude keeping”----as long as Polaris is visible.


The question then becomes, “Did even Viking navigators adopt the Arab Al-Kemal in order to successfully sail to and from Iceland, Greenland and European ports for at least two centuries?” We know that they had engaged in extensive trade with Arab merchants in the Black Sea and the Mediterranean.


There is no remaining physical evidence that they did, so as far as I have been able to discover by researching available literature. It is not surprising that such a simple wooden device would not have survived, or if it had, might well have been dismissed by a non-mariner archaeologist as a child's toy or spindle whorl. Nor can I find reference to such a device available in scientific literature or even the mythology.


But early in my transformation from lawyer to fisherman/mariner I spent some time aboard a very well-found Bering Sea crab-fishing boat owned and skippered by a Norwegian immigrant. There is a sizable Norwegian---now Norwegian-American--community in the Ballard neighborhood of Seattle. Many of them were among the pioneers of the Bering Sea crab fishery, and while I was aboard in 1981 spoke to one another in Norwegian, which no one else could comprehend. Today their descendants simply use satellite telephones for proprietary discussions.


The owner/skipper of the Shelikof Strait  was at that time in his mid-sixties. He had served as a merchant mariner aboard freighters during WWII, on convoys in the North Atlantic, and told me that the captain with whom he'd made several voyages, also a Norwegian, had insisted that the life boats aboard be equipped to sail, be well provisioned, include supplemental foul weather clothing, and a very simple device to allow them to sail to a known latitude with the assumption that abandoning ship would be done without time to gather charts, tables or sextant.  He did not describe it thoroughly, as I probably was not yet as interested in such things as I am now, but he did claim that such devices had been used by his Viking ancestors. Whether that claim was myth born of national pride or based in fact I cannot attest.


But I am compelled to believe that much more than a sun compass would have been essential to successfully completing repetitive landfalls as Viking traders voyaged back and forth between Europe, Iceland and Greenland for centuries. To re-locate Iceland, a substantial but still small island in the middle of the North Atlantic even in excellent visibility, would have demanded some form of “latitude keeping”.  After all, Spanish Manila Galleons sailed just south of Hawaii for centuries and never sighted it. Re-locating an island, once discovered, most likely could not have been done simply with dead reckoning, but would have required the ability to return to that latitude, then run down that latitude until sighting the objective. Whether the device they employed to do so was the “Al-Kemal” or another simple instrument we may never know. But such repeated successful voyages must have involved some form of discerning latitude.


Later, the Portuguese or Portuguese -rained mariners like Columbus, Cabot and Verrizano used what they'd learned at Henry's school at Sagres and used a simple quadrant to get some notion of latitude to enable them to sail by “Latitude Keeping”.  From that evolved the modern sextant, Loran C, Sat-Nav, and today's GPS.