Is the sun shrinking? New analyses suggest a 76-year cycle

One of the intriguing minor controversies in astronomy is the question of whether or not the sun is shrinking. Many astronomers thought the issue died when some investigators showed that the shrinkage was only a misreading of the data. Now it looks as though there may be some size change after all. Indeed, the sun may go through a 76-year cycle of alternating expansion and contraction.

The question was first raised a few years ago by John Eddy of the High Altitude Observatory at Boulder, Colo. Together with Aram A. Boornazian, a mathematician with S. Ross & Co. of Boston, he 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 solar diameter made by observing how long it took the sun's disk to cross a fixed line of sight. Eddy and Boornazian thought they found evidence of about 2 arc seconds contraction in angular diameter a century (36.6 meters a day) as seen from Earth.

At that rate, the sun would contract to a point in 100,000 years and would have been twice its present size 100,000 years ago. That's something neither the geological record nor solar theory could support. Fortunately, other studies soon showed that no such dramatic shrinkage is actually going on. For example, Irwin I. Shapiro of the Massachusetts Institute of Technology studied a different kind of data which are also available for several centuries - transits of the planet Mercury across the solar disk. Shapiro showed that the appearance of dramatic shrinkage was due to inconsistencies in the data Eddy and Boornazian used.

Now there is yet another reassessment, this time by Eddy's Boulder colleague Ronald Gilliland. Using Eddy's data, two sets of Mercury transit data, and records of eclipse observations he finds evidence of real changes in the sun's diameter, albeit on a scale ten times smaller than had been suggested - 0.2 arc seconds per century. Moreover, the changes are periodic with the sun shrinking and growing in a 76-year cycle. The last maximum was in 1911. The next one should come in another few years.

An earlier study by John Parkinson and colleagues at Britain's Mullard Space Science Laboratory had also found some sign of a roughly 80-year cycle using Mercury transit data. However, their study was much less thorough than that of Gilliland. They also found that any long-term trend toward a smaller average radius had to be less than 0.1 arc seconds a century. They concluded the sun's average radius probably is constant. Gillilard thinks that such a small shrinkage may well be going on.

All of this leaves the question Eddy originally raised. Is the sun's changing size, and presumably changing energy output, influencing Earth's climate? It seems that both solar physicists and meteorologists may still have a puzzle on their hands.