OK, I’ll admit it: With so NASA many missions under way right now, it’s hard to keep track of what they’re all up to. That’s why it was a little surprising to learn this week of an intriguing new result from a scientific spacecraft that’s been off my radar screen for some time.
The Reuven Ramaty High-Energy Solar Spectroscopic Imager &mdash called RHESSI, though it was just HESSI before being renamed to honor a pioneering solar physicist — rocketed into space on February 5, 2002. Since then it’s been scrutinizing the Sun’s face with X- and gamma-ray imagers, primarily to probe how energy is released and propagates during solar flares.
Apparently RHESSI has been moonlighting as a solar yardstick, measuring the Sun’s dimensions with unprecedented accuracy. Our star’s spin rate, once every 25.3 days at the equator, causes its midsection to bulge outward ever so slightly relative to its poles. The predicted value of this oblateness is 7.8 milli-arcseconds, about the apparent size of a dime in Boston as seen from San Francisco. It’s only about 0.0001% of the Sun’s diameter — not the sort of deviation that’s easy to detect, let alone measure.
Yet in this week’s Science Express, where Science trumpets articles before they appear in print, a quartet of researchers led by Martin Fivian (University of California, Berkeley) announced that the Sun is a little more oblate than predicted, a hair more than 8 milli-arcseconds. Moreover, when the team combined RHESSI’s measurements with those acquired previously, it found that the polar flattening becomes even more pronounced, by another 10.8 milli-arcseconds, during times of high solar activity.
These results might seem trivial, but solar physicists assure us they are not. The changing girth arises in magnetic ridges on the Sun’s surface that mimic, subtly, the texture of a cantaloupe’s skin. The deviation from a perfect sphere has implications for how the Sun pulls on Mercury, how the solar core is shaped, and perhaps how acoustic waves propagate throughout the solar interior.
Fortunately, the variable oblateness is far too inconsequential to affect predictions for the durations of solar eclipses — except for the most ardent purists. As diehard “umbraphile” Glenn Schneider points out, eclipse calculations usually consider the shape of the solar and lunar disks only to a precision of about 1 arcsecond, 50 to 100 times greater than what RHESSI measured.