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Two Harvard astronomers are among hundreds of scientists from all over the world who are converging on South America this month to study a total eclipse of the sun.
One has encamped at an elevation of 13,000 feet in the Peruvian mountains; the other will be airborne during the brief moments on the morning of November 12 when the moon's shadow will blot out the disc of the sun.
Two other Harvard scientists are also participating in the international study, but from a secure position in Cambridge. They will direct an experiment from here using the information relayed from a tracking telescope at Arequipa, Peru.
Robert W. Noyes, lecturer on Astron omy, left over a week ago to set up camp in a converted railway car in the mountains east of Arequipa.
He will study a phenomenon known as "limn darkening," which is jargon for the gradual decrease in the intensity of light from the center of the sun to its outer edges. By observing into the far infrared, Noyes hopes to obtain information on how the temperatures vary in different layers of the sun's atmosphere.
Donald H. Menzel, professor of Astrophysics, flew to Arequipa last week. Menzel, a seasoned eclipse-observer with well over a dozen expeditions to his record, will be aboard a jet plane as November 12 dawns, at a height that will assure cloud-free visibility regardless of the weather below. Being airborne will also serve to prolong the viewing duration o the eclipse. For ground-based Noyes, totality will last about 90 seconds; but by flying at jet speed in the direction that the sun travels across the sky, Menzel will keep up with the eclipse for up to two and a half minutes.
The Noyes and Menze! expeditions typify the mass influx of astronomers and equipment that awaits an area experiencing a total solar eclipse. A total eclipse provides scientists with their best opportunity to study the outer layers of the sun's atmosphere, and while the phenomenon occurs somewhere in the world about once every year, it always attracts a large scientific audience.
Last week, Jay M. Pasochoff '63, teaching fellow in Astronomy, and James P. Pollack, associate of the Harvard College Observatory, sent detailed directions to the technicians at the Baker-Nunn telescope at Arequipa, and now have nothing to do but wait for November 12.
The experiment Pasochoff and Pollack are directing involves "Baily's beads"-- an astronomical metaphor for the tiny glints of sunlight which appear to ring the moon instants before the achievement of a total eclipse. The "beads" can only be observed near the edge of shadow of totality -- either slightly inside it or slightly beyond it -- and present tracking predictions indicate that the Baker-Nunn telescope will be favorably situated for photographing them.
The phenomenon arises when the last rays of sunlight before totality gleam through the valleys and depressions of the mountainous lunar surface, creating, in effect, a "perforated" ring of sunlight about the edge of the moon. By accurately timing the event, the experimenters plan to collate more precise measurements of the moon's diameter.
Francis Baily, a 19th century English astronomer, first detected the bead-like effect during the eclipse of 1836.
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