The Solar Aspect System of the RHESSI spacecraft scans the limb at the ~4 sec rotation period of the spacecraft, producing a large quantity of precise differential measurements of the solar radius at optical wavelengths (monochromatic at 670 nm). These data provide the most precise determinations of the oblateness prior in particular to the launch of the Picard mission in 2008.

The observation of standing waves in the body of the Sun (helioseismology) provided the first direct way to study the interior of a star. The astrometric shape of the solar limb gives independent constraints on interior structures and flows; the surface rotation predicts an oblate ellipsoid with an equator-pole radius difference of some 8 mas (~0.001%). Here I present the most accurate observations to date of the solar shape, which show a much larger apparent oblateness with an equator-pole radius difference of 13.72± 0.44 mas. This new component can easily be distinguished spatially from the effects of faculae in the active latitude zones.

Comparison with earlier observations suggests a correlation of this excess oblateness with the solar cycle.