Artificial Eclipses

Astronomers, sedentary folk, once got much of their exercise running down solar eclipses. That was the only time they had a good look at the lively goings-on in the sun's atmosphere. But those eclipses, that dragged astronomers to Godforsaken parts of the earth, happened less than once a year, lasted only a few minutes and were often masked by clouds.

Not any more. Last week in Technology Review, Sungazer Walter 0. Roberts of Harvard told how up-to-date astronomers dim the sun enough to get a good look at its atmosphere any time they please.

The light from the sun's surface is so bright that it overwhelms the comparatively weak light from the sun's atmosphere. But the surface light has a "continuous spectrum" (all colors and wavelengths), while most of the light from the atmosphere is concentrated in comparatively few wavelengths.

This difference allows astronomers to photograph the brighter parts of the sun's atmosphere with a "spectroheliograph," a prism spectroscope which casts sunlight of only one color on a photographic plate. The light from the solar atmosphere, glowing in that color, shows in the picture. Most of the dazzling light from the surface, being of other colors, is excluded by the spectroscope.

Another approach is the "coronagraph," developed by Dr. Bernard Lyot of France in 1930. It is a telescope with an internal disc hiding the face of the sun, and specially designed to eliminate glare. Though tricky, it works even better than the spectroheliograph, showing the corona, the faintly glowing halo which surrounds the sun.

Magic Sandwich. Most recent gadget is the "birefringent filter," designed in 1940 by Dr. John W. Evans of Chabot Observatory, Oakland, Calif. It is a multi-decker sandwich of thin quartz plates and sheets of polaroid, which passes only light of a single pure color. Accomplishing the same object as the spectroheliograph, it is much more effective and easier for astronomers to use. When built into a coronagraph, it lets the complexities of the sun's atmosphere be seen in all their terrifying glory.

High (11,500 ft.) in the Rockies near Climax, Colo., Dr. Roberts watches the sun through the thin, clean air and through Harvard's coronagraph, with its birefringent filter. He finds the sight a perpetual three-ring circus. From the dazzling surface of the sun (well screened by his gadgets), enormous gaseous solar "prominences" leap in graceful arcs at several hundred miles per second, driven by unknown forces (see cut). Little "spicules" (big enough to be seen at least 93 million miles away) jab up and fall back in four minutes. The ghostly corona waxes and wanes.

These phenomena, says Dr. Roberts, influence life on earth. Their varying radiation and blasts of speeding electrons affect the earth's upper atmosphere, are felt in the weather, cause the electrical disturbances which ball up radio and wire communications.

Not many' of their effects are fully understood. But, during the war, Dr. Roberts' sun photographs, even before the plates had been washed, were described in code and flashed to Washington and Britain. By looking at the sun via Climax experts could tell, days in advance, when an electrical barrage would block war-vital messages.

This file is automatically generated by a robot program, so reader's discretion is required.