The solar cycle and the Sun’s shape

Like all spinning, fluid celestial bodies, the Sun is oblate. But its equatorial radius is just a few parts per million longer than its polar radius, which makes the solar oblateness very difficult to measure. Robert Dicke and coworkers tried hard half a century ago, hoping to find a departure from spherical symmetry large enough to reconcile Dicke’s alternative theory of gravity with the known precession of Mercury’s orbit. Later measurements made it clear that the Sun’s oblateness was much smaller than Dicke’s theory required. But inconsistency among the later measurements hinted at a variation of the oblateness that might be correlated with the 11-year cycle of sunspot abundance and other solar variables. Now a team led by Dicke’s former student Jeffrey Kuhn has used NASA’s orbiting Solar Dynamics Observatory , launched in 2010, to measure the shape of the Sun’s limb with high precision and look for temporal variation. They report that, in terms of angles subtended at Earth, the Sun’s equatorial radius exceeds the polar radius by 7.2 ± 0.5 milliarcseconds. (The whole solar diameter subtends about half a degree.) Furthermore, over two years of observing, during which the sunspot abundance rose from minimum to near maximum, they found no significant evidence of variation in the limb’s shape. And the apparently constant oblateness turns out to be three standard deviations smaller than that predicted by solar-rotation models based on helioseismic data. (J. R. Kuhn et al., Science, in press, doi:10.1126/science.1223231 .)—Bertram Schwarzschild