Saturday, September 29, 2012

The Sun is Too Round

Back in the 19th century it was observed that the orbit of Mercury deviated from the predication of Newton's law of gravity - the precession of Mercury's perihelion. One simple explanation would have been that the sun was oblate: flattened at the poles and bulging at the equator. However the sun is much too round for that to work, instead Einstein explained most of the anomaly in Mercury's orbit with a modification of Newton's gravity - his theory of General Relativity. However there was still a small discrepancy between Einstein's theory and the actual orbit of Mercury which was expected to be explained by some flattening and bulging of the sun from perfect roundness. However, the actual shape of the sun has been mighty hard to measure to the required accuracy - until recently. Alas, it appears that the sun is still rounder than expected. How Oblate Is the Sun? by Douglas Gough in Science is a review of the history of this problem and The Precise Solar Shape and Its Variability by Kuhn et. al. in the same issue is a report on recent space-based measurements.