Europeans did most of their sea trading along coasts that were near them, and mostly in an east-west direction. If they were out of sight of land, it was usually not for more than a few days. There was no need, and therefore no interest, in measuring distances north and south.
The Arabs, however, traded along the dangerous shoals (shallow waters) and strong currents off the coast of East Africa which ran from north to south, and as far off as India, out of sight of land most of the time. It was important for them to know how far north or south they traveled along an unseen coast before it was safe to turn toward that coast and make their landfall. The device they developed was called the Ka-Mal, which means “guide” in Arabic. Though very simple and “low-tech,” it was used by the Arabs of East Africa and the Red Sea as recently as the 20th century. We don’t know when it was developed, but sometime after 900 CE, a time we know the Arabs had the Astrolabe. It also seems the Arabs developed the Ka-Mal from a similar Chinese invention.
The Ka-Mal in its simplest form was a piece of wood; the navigator sighted the horizon at the bottom of the wood and Polaris at the top. When everything lined up, the ship was at the right Latitude to turn toward the city of their arrival. There would be a different piece of wood for each port. In time, the multiple pieces of wood were replaced by a single piece with a hole in the middle through which a string was fixed. A knot in the string, placed between the navigator’s teeth, would then set up the correct proportion of distance from the eye, and an alignment of the horizon and Polaris.
Each knot in the string represented the latitude of a port they wished to make, but they did not use that term or use a latitude-longitude system. Vasco da Gama, the Portuguese explorer, was introduced to the Ka- Mal when he visited India in 1498. This concept was taken up by the Europeans in the 1500s and led to their developing the Cross-Staff.
