Used to indicate the distance traveled north or south, the kamal was used to guide Arab ships to port. It led to the later development of the cross-staff.
In the Medieval period, 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 kamal, which means “guide” in Arabic. Though very simple and “low-tech,” it has been 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. It seems that the Arabs developed the kamal from a similar Chinese invention.
The kamal uses the position of Polaris (the North Star) in the sky to help a sailor determine his latitude. The height of Polaris in the sky depends on a person’s location on Earth, so by measuring the height of Polaris, a sailor could tell where on Earth he was standing. The kamal was a simple piece of wood that a navigator could hold up at arm’s length in front of his face. The navigator would line up Polaris (the North Star) with the top of the wood, and the horizon at the bottom of the wood. When both lined up, the navigator would know that his ship had reached the latitude of its home port, and it was time to turn inland. Using this method, a navigator would need a different-sized piece of wood for each port he wanted to visit.
In time, navigators developed a kamal that could help them find many different ports. A rope with several knots tied in it was added to the kamal. Each knot represented a specific port. By holding a knot between his teeth and stretching the rope, a navigator could use the same piece of wood to measure different heights and find his way to different cities.