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Polar Navigation and the Sky Compass

You are a Viking captain, 900 years ago. You are sailing westward to Greenland -- you hope. It's getting late in the sailing season, and during the long polar twilight, with neither sun nor stars visible, directions are impossible to determine. Even in the daytime, the sun is often hidden by fog. You've heard vague rumors of a strange black rock that points to the north, but you've also heard that ice somehow disturbs its workings. Surely a sunstone is more reliable. If only yours had not been stolen when you stopped for supplies in Iceland!

Jump forward in time, to the middle of the twentieth century. Now you are a navigator for Scandinavian Airlines, pioneering commercial flight across the North Pole. Nine centuries of technological development have done little to assist in the solution of your Viking ancestors' problem. The magnetic compass is no longer a vague rumor, but its problems at high latitudes are real, although not caused by ice. Celestial navigation when the sun is just below the horizon is still impossible. In fact, the problem has become worse, both because you are now traveling over the pole, where twilight can last for a week or more, and because the airplane is moving so fast that even an hour or two of flight without directional control can bring disaster. The solution to your problem is the sky compass -- the functional equivalent of the Viking sunstone.

Both the sky compass and the sunstone work by determining the direction of polarization of the sky overhead. Light scattered by air molecules is polarized, and the direction of the polarization is at right angles to a line to the sun. If the sun is just below the horizon in the west, for instance, the sky overhead will be polarized along the north-south direction. If the sky overhead is clear, you can determine the direction of the sun by determining the direction of polarization of the sky directly overhead. (Actually, the sun could be at either of two points opposite each other. But there is usually enough difference in the brightness of the sky in the two directions to determine which one hides the sun.)

The Viking sunstone was probably a natural crystal of cordierite. This mineral, a silicate of magnesium, aluminum, and iron, has some similarity to modern polarizing filters in that it absorbs light of different polarizations differently. In cordierite, however, the absorption also depends on the color of the light. A natural crystal of this mineral looks somewhat like a quartz crystal, but would rarely be as large as the size of your little finger. The stone absorbs blue light more strongly than yellow if the light is polarized along the long axis of the crystal, and yellow light more than blue if the polarization is across the long axis. If the crystal is held overhead and rotated, its color changes from yellow to blue when the long axis is pointed toward the sun. Clouds on the horizon cause no problems, as long as the sky directly overhead is clear.

The modern sky compass is based on man-made polarizing filters. It started with the work of Dr. A. H. Pfund, who was studying the polarization of scattered light from the sky in 1944. Commander T. D. Davies of the U.S. Navy, who heard of the device three years later, was well aware of the problems of twilight navigation, and was instrumental in getting four experimental instruments made for the Navy. Many of the Air Force's Ptarmigan flights to the North Pole in the 1950s used the sky compass.

SAS made further improvements and used the instrument for many years on their polar flights. The sky compass had a star- or cross-shaped insert of a material that rotated the plane of polarization of the incoming light, as well as a polarizing filter. When the instrument was properly aligned with the hidden sun, the cross disappeared.

The sky compass is rarely used today, as it has been superseded by reliable gyrocompasses, inertial navigation systems, and most recently, satellite positioning systems. But the most interesting aspect of this might be summed up by the old proverb that there is nothing new under the sun. Both devices grew out of observable natural phenomena. Both were recognized as useful solutions to the same problem. But they were almost a thousand years apart in time and in technology.