Lighted by Regulus

At Madrid's National Observatory one afternoon last week, a group of U.S. astronomers peered at the sky with astronomers' telescopes that can see planets and stars in bright daylight. Headed by Dr. Allen Hynek of the Smithsonian's Cambridge Astrophysical Observatory, the scientists were in Spain to take full advantage of a rare event. The planet Venus, 55 million miles from the earth in the solar system, was passing directly in front of the bright star Regulus in miniature eclipse, and though the two were 400 trillion miles apart (67 light-years), the star's light would enable them to poke deep into the atmosphere of Venus. It was an opportunity that might not occur again for 1,000 years.

As the hour of 2 p.m. universal time* approached, Venus looked like a yellow half-moon against the sky background, and Regulus, greenish in hue, was approaching the rim of its disk. The occultation was to start at 2:21. The minutes passed; the star edged closer to the invisible rim of the planet. "No change, no change," chanted Hynek into a tape recorder while an assistant read off the time. "Gosh, there--it seemed to go. It's definitely going, going. It's gone." Eleven minutes and 4.8 seconds later, Regulus reappeared from behind the bright edge of Venus. The star seemed to struggle to get away, clinging for five or six seconds before drifting clear.

First Magnitude. Those brief seconds of gradual fading and slow reappearance were the reason for all the excitement. When the earth's airless moon occults a star, the star winks out instantaneously. But Venus has an abundant atmosphere, and so a star that it covers fades slowly, its light changing and diminishing like the setting sun. Careful observation is sure to tell volumes about the Venusian atmosphere, its density at various heights, its temperature and chemical makeup.

Astronomers, who consider the planets as prospective real estate for the space age, have longed for years to see Venus occult a bright star. But such events are extremely rare. Venus looks big because of sunlight reflecting brightly from its faintly yellow cloud deck; actually, to earth-bound observers its disk is never larger (usually much smaller) than a golf ball seen from a distance of 500 ft. As the tiny sphere creeps slowly across the star field, it occasionally covers a faint star, but not once since the invention of the telescope 350 years ago has it covered anything like Regulus, a star of the first magnitude.

Tough Calculations. The experts might have missed the event altogether had it not been for British Astronomer Gordon E. Taylor, a former amateur without university training, now employed at the Royal Greenwich Observatory. At first, some of the pros doubted Taylor's calculations, which were published in January; the paths of two such remote bodies are very tough to calculate accurately. Only when the august Harvard College Observatory confirmed Taylor's calculations did the occultation of Regulus become a serious concern of world astronomy. The U.S. was ruled out as a major observation point because Venus and Regulus would be close to the eastern horizon with the sun above them. With help from the U.S. Air Force and Boeing Airplane Co., Harvard sent trained observers with elaborate light measuring devices to France, Spain, Italy and Lebanon; other astronomers in South Africa and Asia set up watch.

A few were foiled by clouds, but many reported clear skies. The films, tapes and other records that they made do not look like much, but with careful analysis in the next few months a better picture of the Venusian atmosphere will be assembled. When the first space traveler from earth attempts to explore Venus, he will know much about what to expect, and for that he can thank winking Regulus so many trillion miles away.

When Jupiter occults a bright star, astronomers will learn a lot about its atmosphere, which is probably thousands of miles deep and boiling with enormous storms. Until that happens, which may not be soon, they must be content with shreds of information picked up in other ways. Jupiter sends out fairly powerful radio waves, which first seemed to indicate that the temperature of the atmosphere is surprisingly high: up to 1,000DEG F. Odder still, the temperature apparently fluctuates over a wide range.

Recently Dr. Frank Donald Drake of the National Radio Astronomy Observatory at Green Bank, W. Va. theorized that Jupiter's radio waves do not come from the atmosphere at all but from a vast Jovian version of the double doughnut of Van Allen radiation that surrounds the earth. Ionized particles from the sun zigzagging back and forth in Jupiter's magnetic field must be sending out "synchrotron radiation" like the circling particles in a synchrotron. The theory alerts future space explorers to steer well clear of Jupiter. If their ship should cruise too close, they might be fried by Van Allen radiation 100 times as strong as that surrounding earth.

* The time commonly used by astronomers, which navigators call Greenwich mean time.

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