The sun is smaller than it was 265 years ago and appears to be shrinking even more.
There's no panic in the solar physics community about it but there's also no doubt that the sun isn't the same. To be more precise, the sun's diameter is believed to be about 700 kilometers less than it was in 1715, the last year of the 77-year reign of France's King Louis XIV.
"It's not much of a shrinkage, about one-twentieth of 1 percent," said the Goddard Space Flight Center's Dr. Sabatino Sofia, who compared observations of the total solar eclipse of 1979 with those made by British astronomer Edmund Halley of the total eclipse of 1715, "but our data nonetheless suggests the whole ball has shrunk."
Other studies of the sun at the California Institute of Technology's Mount Wilson Observatory show that the solar disc has lost 43 kilometers from its middle in the last six years, a tiny lost but still a shrinkage.
"We've seen a steadily decreasing diameter for the last six years," said Mt. Wilson's Dr. Robert F. Howard, who measured the sun's diameter with a photoelectric scanning device every day during that period. "We don't know why it's been shrinking but we think it's got something to do with the 11-year sunspot cycle."
So does Sofia, who notes that in 1715, when the sun was 700 kilometerss wider across than it is now, the so-called Maunder Minimum came to an end. The Maunder Minimum was an 80-year period during which sunspots almost disappeared from the solar surface.Coincidentally, the Earth suffered its coldest weather in 1,000 years.
"It's not unreasonable to suspect that a slight expansion of the sun took place around 1715 or just before," Sofia said. "The expansion could have produced a slight heating of the sun that brought the 'Little Ice Age' to an end."
A shrinking sun could mean a drop in solar luminosity, the total amount of energy the sun radiates day in and day out. Scientists at the University of Denver have reported that solar luminosity is down. So have rocket scientists attempting to confirm the Denver results. Measurements made from mountains in Chile and southern California in the last year suggest that the amount of radiation pouring off the sun is less than it was 10 years ago.
"Nobody's saying we're moving into another Little Ice Age," Goddard's Sofia said, "but our ultimate goal in all this is to understand enough of what's going on to acquire a predictive capability that could tell us if the Earth is facing some kind of climate change."
There are more measurements than those made by Sofia and Howard to tell us the sun has been shrinking.
Dr. John A. Eddy of the National Center for Atmospheric Research in Boulder, Colo., has done an exhaustive survey of the records kept at Britain's Royal Observatory in Greenwich of the time it took the sun to cross the prime meridian at high noon every day the sun was visible.
Eddy covered the records from 1851 to 1953, and while there were years when the sun was visible only 40 times there were also years when it was tracked 150 times. Eddy calculated that the sun lost more than 1,000 kilometers in diameter that 102 years but received such a negative reaction from his fellow astronomers at a meeting last year that he still hasn't published his findings. a
"It's a touchy issue, we've received a real stubborn reaction to it," Eddy said the other day from his laboratory in Boulder. "But no matter how many adjustments we make to our data for things like cloudy or murky days, we still get a shrinking sun."
Eddy said that it's just possible the sun's outer envelope of gas expands and contracts all the way through time. Eddy said that 16th Century records suggest the sun was bigger then than it's been at any time since.
"There was a solar eclipse in 1567 that was described in very rich detail by Christopher Clavius, an astronomer who had experienced a previous solar eclipse," Eddy said. "What Clavius described was not a total eclipse but an annular eclipse where parts of the sun were never covered by the moon. This suggests a larger sun."
While the early reaction of most astronomers to the shrinking sun reports was skeptical, many of them are beginning to accept the notion that they may indeed be true. They are especially impressed with the work done by Sofia and Dr. David W. Dunham of Computer Sciences Corp. in Silver Spring, who devised an ingenious way to measure solar shrinkage.
Sofia and Dunham placed observers at each end of the path of totality in the 1979 solar eclipse, which took place over the Pacific Northwest. The observers then measured the time it took for the sun to travel the entire path. Knowing the exact distance from the Earth to the moon and sun, the scientists then calculated the size of the sun.
The scientists then compared their observations with those made by renowned British astronomer Halley of the 1715 solar eclipse. Halley had hundreds of observers strung out all over England, not to observe the path of totality to measure the size of the solar disc but to see how well he could predict its path ahead of time.
Sofia believes the sun expands and contracts over time and that the mechanism for expansion and contraction is the changing magnetic field of the sun, which triggers the 11-year solar flares wax and wane. Sofia thinks that what expands and contracts is the outer 25 percent of the sun, where columns of hot gas are constantly moving up and mountains of cooler gas descending toward the sun's interior.
"My guess is that what we saw in 1715 was a large and unusual expansion of that outer envelope," Sofia said. "So that what we have now is not so much a small sun as it is a sun that's a little smaller than the large sun we had in 1715."