  The general relativistic (Mercury-type) periastron advance is calculated here
for the first time with exquisite precision in full general relativity. We use
accurate numerical relativity simulations of spinless black hole binaries with
mass ratios 1/8 < m1/m2 < 1 and compare with the predictions of several
analytic approximation schemes. We find the effective-one-body model to be
remarkably accurate, and, surprisingly, so also the predictions of self-force
theory [replacing m1/m2 --> m1m2/(m1+m2)^2]. Our results can inform a universal
analytic model of the two-body dynamics, crucial for ongoing and future
gravitational-wave searches.
