Celestial Navigation (II): kemal and the North Star

We continue our journey to the south.

When the clouds allow it, the sky gives us a spectacle that will never tire us: the dance of the stars.

Only one remains motionless throughout the night: the North Star, Polaris. It shines in its exact place, five times to the left of the line formed by Dubhe and Merak, the "Pointer stars" of the Big Dipper.

Any attempt to understand astronomical navigation in its full dimension involves a journey back in time. It is an ancient art that comes from an era when there was no possible alternative. It is the result of a necessity.

That fixed point in such a mobile and changing universe like the ocean was invaluable when man began to venture far from the known coast and was in need to orient himself. However, the importance of the North Star for ancient navigators goes even beyond pinpointing the north with precision.

Most of us are unaware of the important information revealed by its height, its angle above the horizon. And this is where the symmetry and beauty of the sphere come into play.

At the North Pole, the latitude is 90º and Polaris appears high, 90º above the horizon.

At the equator, the latitude is 0º and Polaris is on the horizon, 0º above it.

This direct relationship applies regardless of latitude, at any point in the Northern Hemisphere.

This means that, once the North Star has been identified in the night sky, all you have to do to know your latitude is to measure its angle above the horizon. It is as simple as that.

Two examples:

If one manages to measure the altitude of the North Star in Barcelona, the resulting angle will be 41º20'. That is exactly your latitude.

Makkum, birthplace of the Vinson, is 12º of latitude further north than Barcelona, so the North Star is 12º higher in the firmament in Makkum than it is in Barcelona.

To accurately measure the height of Polaris requires practice and using a sextant, but this principle can be demonstrated even without instruments, with bare hands: With the arm well stretched, the open palm of our hand covers about 20º; the closed fist 10º; the thumb 2º, and the little finger 1º. Do the test yourselves and you will see how you calculate your latitude with an error of only 2 or 3 degrees.

It was Vasco da Gama, returning from one of his expeditions at the end of the 15th century, who described and introduced in Portugal the kemal, an astonishingly simple instrument, considered the first instrument of astronomical navigation and precursor of the sextant.

At that time the Portuguese used to hire Arab pilots to guide them in their expansion in the Indian Ocean.

The kemal consists of a small wooden rectangle with a hole in the center, from which a thin rope with knots comes out. Each knot corresponds to a latitude, to a usual port of call.

This simple instrument became a true navigational chart, to such an extent that what distinguished an experienced pilot was the kemal he wore around his neck because he had made it himself over the years, tying a new knot each time he visited a port for the first time.

Its operation was simple. The pilot put the knot between his teeth and tightened the rope, the lower edge of the kemal had to be aligned with the horizon and the upper edge with the North Star.

Let's give an example of how it worked: Suppose a ship was in the middle of the Arabian Sea and wanted to head for Bombay, India. It sailed a northerly course following the North Star and every night the pilot went out on deck to measure its height. He put the knot corresponding to Bombay between his teeth, and stretched the rope until the lower edge of the kemal was aligned with the horizon. If the star was hidden behind the upper edge, Bombay was still to the north and he would order to keep that course.

On the night when the star coincided with the upper ridge, the pilot ordered to set a course to the west before the astonished eyes of the sailors, who could not understand how he could know that it was time to turn and set course to port…

Just when we were sailing in Portuguese waters, land of great navigators and explorers, we built a rudimentary kemal with the means on board.

It serves as a humble tribute to the intrepid navigators who precede us.

Jordi Griso

Crew