New look at 'shrinking' Sun

The intriguing notion that the Sun is shrinking isn't holding up under rigorous scrutiny. That's bad news for film writers who may have thought they had a new disaster scenario last year when a solar physicist, John Eddy, reported a shrinkage rate that would have reduced our Sun to a point in a mere 100,000 years. But it does reassure theorists who realized that the phenomenon, if true, would mean they had no understanding at all of how the Sun works.

They needn't worry. Irwin I. Shapiro of the Massachusetts Institute of Technology has restudied the question from a different viewpoint and finds no evidence for such rapid shrinkage. Eddy, he concludes, has been misled by the inherent uncertainties of old observations that, at best, were made at the limits of what could be seen from Earth, even by skilled observers.

Eddy, who is with the National Center for Atmospheric Research, and Aram A. Boornazian, a mathematician with S. Ross & Co. of Boston, had studied records of Britain's Royal Greenwich Observatory and the US Naval Observatory that go back as far as 1750. These data are direct measurements of the Sun's diameter made by observing how long it took the solar disk to cross a fixed line of sight. Using data sets collected between 1836 and 1953 at Greenwich and since 1846 by the Naval Observatory, they believed they had evidence of shrinkage of about 2 arc seconds in angular diameter a century (36.6 meters a day) as seen from Earth.

At that rate, the Sun would be a point in 100,000 years and have been twice its present size 100,000 years ago. Eddy and Boornazian weren't suggesting any such marvel. But Eddy did note that a temporarily fast rate of shrinkage could imply an unsuspected variability in the Sun that, in turn, might affect climate.

A contemporary but independent study, by S. Sophia and others at the Goddard Space Flight Center and Louisiana State University, found only about 0.25 seconds of variation in solar diameter from 1850 to 1937, with no systematic shrinkage. But Eddy and Boornazian were confident of their analysis, which if confirmed would be the most important solar discovery of the century.

Shapiro's work now confirms the findings of Sophia and his colleagues instead. As he recently explained in Science magazine, Shapiro used a different kind of data -- transits of mercury across the solar disk. These also have been recorded for centuries. And, Shapiro believes, they are more reliable when used to deduce solar diameter changes than the direct observations. He too finds no more than about 0.25 seconds' variability, with no systematic shrinkage.

The thoroughness of this study casts strong doubt on Eddy's thesis, if it doesn't utterly demolish it. From the viewpoint of science, both Eddy and his critics have done solar physics a service by encouraging restudy of valuable, old data. But film writers will have to look elsewhere for inspiration.

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