Big Blackout

At Bocayuva, Brazil this week, an eager group of U.S. scientists were happily tuning up their instruments (everything from thermometers to a 6-29). They were waiting for the moon to eclipse the sun on May 20. The scientists had picked Bocayuva, 400 miles north of Rio de Janeiro, as the spot most likely to have clear skies on the big day. If clouds should blind the ground instruments, airplanes will take off early to observe what they can from high altitudes.

Totality will last three minutes, 48 seconds--the longest solar blackout since 1940 and until 1955. The National Geographic Society and the U.S. Army Air Forces, which are running the show, believe that this will be the best observed eclipse in history. When the moon's shadow races across Bocayuva, there are many questions to be asked--some new, some old.

Check on Einstein. An old question is Einstein's Theory of Relativity. One way to check on Einstein is to measure how much a star's light rays are bent by gravitation when they pass close to the sun. These measurements can be taken effectively only during a solar eclipse.

Thus far, observations during eclipses have confirmed Einstein. But the exact amount that a star's light bends has varied from eclipse to eclipse because of changes in the density of the upper atmosphere. This year, the scientists, with their passion for precision, will study the atmosphere right up to its top, with airplanes and radiosonde balloons.

Hole in the Sky. A comparatively new problem is the effect of the moon's shadow on the ionized layers in the earth's upper atmosphere. These layers have a profound influence on radio transmission: they bounce some radio waves back and allow others to pass through. Since the ionized layers are caused by ultraviolet light from the sun, they presumably change their character when the moon's shadow knocks a hole in the sun's radiation. Long before the eclipse, begins, the scientists will have radar-like radios of various wavelengths trained upon each layer.

This experiment is particularly engrossing to the Army. Because the fearsome "guided missiles" of future wars will operate through and above the ionized layers, the Army wants to know more.

A few scientists will keep their eyes on the moon, clocking with fine exactitude the four "contacts" as its disc passes over the sun. Their observations should refine the figures about the moon's orbit, and about the solar system in general.

Look at the Sun. Other specialists will watch the sun. During totality, the sun's glowing corona will show up in all its splendor, to be photographed many times in both black & white and in color. The scientists hope that they can work out the suspected relation between sunspots (now near a maximum) and changes in the corona.

Airborne scientists will observe the effect (if any) of the eclipse on cosmic rays. Cosmic rays are not supposed to come from the sun, but they may be influenced somehow by its sudden blackout. Airborne, too, will be Army meteorologists, watching temperature changes in the atmosphere at all levels as the moon's cold shadow sweeps across Brazil. If the weather is good, they ought to get a gorgeous silhouette picture of the earth's satellite.

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