PerspectiveAstronomy

How Oblate Is the Sun?

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Science  28 Sep 2012:
Vol. 337, Issue 6102, pp. 1611-1612
DOI: 10.1126/science.1226988

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Summary

At the end of the 19th century there was much concern over Le Verrier's realization that the orbit of Mercury differed from that expected from Newtonian physics. After taking account of perturbations from other planets, there was an unexplained residual precession of the elliptical orbit that amounted to just 43 arc sec per century. It was pointed out by Newcomb that this residual precession might be explained by the Sun being oblate. Einstein then demonstrated that his new general theory of relativity accounted for almost all of the precession, assuming the Sun to be precisely spherical. Only a small 0.2% of the original discrepancy then remained to be explained otherwise, presumably by an oblateness caused by rotation of the Sun. The most natural way to determine the oblateness is simply to measure the apparent shape. However, despite many attempts over more than a century, that has not been possible with the required precision. The reason is that ground-based observers must contend with variations in the refractive index of Earth's atmosphere, which distort the image of the Sun. Only with instruments in space has it been possible to approach a useful measurement (1). On page 1638 of this issue, Kuhn et al. (2) present results from the Heliospheric and Magnetic Imager (HMI) on NASA's Solar Dynamics Observatory (3), indicating that the Sun appears not to be as flattened as it should be.