Science: Crab Compass

Some scientists are neatness itself, but Professor Talbot H. Waterman works in a wonderful mess. His room at Yale’s Osborn Zoological Laboratory is a tangle of wires, tubes, electrical equipment, optical instruments, pipes, tools and gadgets. And all over the place crawl the stars of the show: live horseshoe crabs. Dr. Waterman is trying to find out how arthropods (crabs, insects, etc.) navigate. The Office of Naval Research is so interested that it has him under contract.

Scientists know that certain arthropods, including horseshoe crabs and bees (TIME, Jan. 1), can steer by the sun even when they cannot see it. All they need is a patch of blue sky. The light that comes from it is partially polarized,* and the direction in which the light vibrates shows the position of the sun. So the bees and crabs, whose eyes are sensitive to polarity, have only to look at the sky. It tells them where the sun is; then they steer by the sun, whether they can see it or not.

Dr. Waterman, who discovered the peculiar talents of the horseshoe crab’s eyes, is now trying to find out how the eyes work. He dissects them under a microscope, attaches their optic nerves to delicate electrical instruments, and measures their responses to light of varying polarity. He removes their tiny lenses and measures their optical properties.

The reason the Navy is interested is the baffling problem that airplane navigators encounter near the North Pole. The magnetic compass isn’t much good because of the nearness of the shifting magnetic pole. In broad daylight the navigators can steer by the sun, at night by the stars. But during the long polar twilight they can see neither sun nor stars.

A good solution for the problem would be a simple, accurate instrument to measure the polarity of the twilight sky and reveal the position of the sun below the horizon. Then the sun could be used to steer by, just as if it were visible. If Dr. Waterman’s work is successful, U.S. pilots may some time steer across the North Pole, high above the overcast, guided by an instrument patterned on the eye of a horseshoe crab.

*Direct light from the sun vibrates equally in all directions. Light scattered by particles in the atmosphere vibrates more in some directions than in others.