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Lunar laser ranging (LLR) data and Apollo seismic data analyses, revealed independent evidence for the presence of a fluid lunar core. However, the size of the lunar fluid core remained uncertain by ±55 km (encompassing two contrasting 2011 Apollo seismic data analyses). Here we show that a new description of the lunar interior's dynamical model provides a determination of the radius and geometry of the lunar core‐mantle boundary (CMB) from the LLR observations. We compare the present‐day lunar core oblateness obtained from LLR analysis with the expected hydrostatic model values, over a range of previously expected CMB radii. The findings suggest a core oblateness ( f   c  =(2.2±0.6)×10 −4 ) that satisfies the assumption of hydrostatic equilibrium over a tight range of lunar CMB radii (   R  CMB   =  381  ±  12   km). Our estimates of a presently relaxed lunar CMB translates to a core mass fraction in the range of 1.59–1.77% with a present‐day free core nutation within (367±100) years.

Observational Constraint on the Radius and Oblateness of the Lunar Core‐Mantle Boundary