The sun is the roundest natural object ever precisely measured, astronomers say.

Astronomers have sought for centuries to see if and how our backyard star's roundness changes. That's because even slight changes in the sun's shape can alter its brightness and, as a result, influence weather and climate on Earth.

But getting a read on the sun's shape from the ground is hard, due to turbulent air that refracts light and generates inaccurate measurements.

Spacecraft eventually worked around that issue, yet poor image resolution became the next hurdle: Without a crisp view of the rotating sun, it's difficult to improve existing measurements.

"The sun is very, very round, so it's difficult to measure any deviations in that roundness," said study leader Jeffrey Kuhn, a solar researcher and physicist at the University of Hawaii.

"It's only been in the last few years that we've been able to make decent shape observations."

Those advances are due in part to NASA's Solar Dynamics Observatory (SDO), which launched in February 2010 and has some of the best cameras fixed on the sun. So when Kuhn earned some time controlling the spacecraft, he got to work.

Spin Me Right Round

Kuhn said four things could toy with the roundness of the sun on human timescales: gravity, rotation, magnetism, and turbulence under its surface. (See pictures of solar storms in National Geographic magazine.)

Previous experiments had ruled out the possibility that gravity and rotation—the sun's surface spins about once every 25 days at its equator—have any year-to-year, month-to-month, or day-to-day effects. Later studies looked at magnetism, which drives the sun's 11-year cycle of magnetic activity.

The magnetism measurements weren't conclusive, though, so Kuhn and his team commanded SDO to take roughly 50,000 high-resolution images over the course of two and a half years.

If the sun were a meter-wide (3.3-foot-wide) beach ball, Kuhn said, the variation in the sun's shape from the highest to the lowest point would be about 17 microns—less than the width of a fine human hair, according to the SDO measurements.

The new measurements hint that magnetism may not influence shape, even though SDO recorded the sun during an increasingly active part of its cycle—one that has triggered massive sunquakes, tornadoes, solar flares, and more, said Kuhn, whose study appears in this week's issue of the journal Science. (Related: "New 'Sunquake' Trigger Found: Huge Solar Belches.")

In fact, the sun's shape was rounder than predicted by any computer model. So that leaves turbulence as a culprit.

Changes in the sun's roundness "may be because of plasma turbulence below the sun's surface, which has a chaotic movement like a pot of boiling water," Kuhn said. "What it really means is that we don't understand turbulence in the outer layers of the sun."

Round Data to Solve Climatic Mysteries?

Astrophysicist Philip Goode, of Big Bear Solar Observatory in California, has studied the sun's shape for 30 years but wasn't involved in the new work. He said the new measurements could help rule out shifts in roundness as a culprit in climatic mysteries of the past.

"People have argued that there's a connection to the sun's shape, and its brightness, to climate events," Goode said. (See more sun pictures.)

The Little Ice Age is one example, he noted. During that cool period, "people had winter festivals on the River Thames, enjoyed Baltic trade between Sweden and Poland, and so on. But the reason for that event is less likely to be as simple as a dip in brightness."

Study leader Kuhn said his team is going to update computer models of the sun's cycle to see if and how the highly accurate shape affects their behavior.