Science: In Arctic Twilight

U.S. airmen flying the far north in search of weather data have often been bedeviled and bewildered by the arctic twilight. During the long arctic winter, the navigators of the 375th Squadron, at Eielson Airforce Base near Fairbanks, Alaska, had no trouble. They used special “grid” maps* and flew by the stars, visible all the time. During the arctic summer, they flew by the never-setting sun.

Blinding Twilight. But during the spring and fall, both the sun and the stars often failed them. The sun remained invisible below the horizon for many hours each day, but it gave enough light to blot out the stars. When a weather-watching B-29 started out for the North Pole, a standard mission for Squadron 375, it had to take off during a particular 15-minute period in each 24 hours. If it flew at any other time, it could not get to the pole and back without having to pass through a broad belt of blinding twilight.

Last week Squadron 375 was sure it had the twilight problem pretty nearly licked. One gadget it has found useful is the Pfund† sky compass which polarizes light reflected from the sky—and points to the spot on the horizon directly above the invisible sun. When used with the proper tables, the sky compass gives the direction in which the plane is flying.

Downward Pull. An even more promising instrument is a special version of the ancient magnetic compass. Ordinary magnetic compasses are of little use in the far north. Their needles do not swing normally, but often try to point almost directly downward toward the magnetic pole a few hundred miles away. The “flux-gate” compass, which uses coils instead of needles, eliminates the downward pull and shows only the small horizontal pull toward the magnetic pole. The compass does not point north, of course. Since the magnetic pole is many miles south of the geographical North Pole, the compass often points almost south. But newly revised magnetic charts allow the navigators to interpret such weird readings accurately.

There are still some problems, but already Squadron 375 can laugh at the treacherous twilight. Every other day a stripped-down B-29 takes off on a 3,500-mile “Ptarmigan flight” to the pole and back. So far, no plane has been lost on the ice cap. Married officers often save bits of in-flight lunch and bring it home to their kids as a present from Santa Claus. Lots of mementos (e.g., flags) have been dropped on the pole itself. Cracked one pilot: “A few more drops and we’ll be sent back in to police up the area.”

*With a “grid” or rectangular network of lines superimposed on the conic charts (with closely converging meridians) which are generally used near the pole.

†Based on investigations by the late Professor August Herman Pfund of Johns Hopkins University.