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The non-Newtonian perihelion precession for the asteroid Icarus has been deduced from optical observations and found to agree with the predictions of general relativity to well within the estimated uncertainty of 20%. If observations are made this June when Icarus passes close to the earth, this uncertainty could be reduced to 8%. With such new data a marginally useful bound could also be placed on the quadrupole moment of the sun's gravitational field.
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Shapiro et al. (1968) studied this question.
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