Measurements of echo delays of radar signals transmitted from Earth to Mercury have yielded an accurate value for the advance of the latter's perihelion position. Given that the Sun's gravitational quadrupole moment is negligible, the result in terms of the Eddington-Robertson parameters is $\frac{(2+2\mathrm{}\gamma -\beta )}{3}\simeq 1.005\pm 0.007$, where $\gamma =\beta =1\mathrm{}$ in general relativity, and where 0.007 represents the statistical standard error. Inclusion of the probable contribution of systematic errors raises the uncertainty to about 0.02.

• Received 10 April 1972

DOI:https://doi.org/10.1103/PhysRevLett.28.1594