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The mean anomaly at epoch $\eta$ is one of the standard six Keplerian orbitalelements in terms of which the motion of the two-body problem is parameterized.Along with the argument of pericenter $\omega$, $\eta$ experiences long-termrates of change induced, among other things, by general relativity and severalmodified models of gravity. Thus, in principle, it may be fruitfully adoptedtogether with $\omega$ in several tests of post-Newtonian gravity performedwith astronomical and astrophysical binary systems. This would allow to enhancethe gravitational signature one is interested in and to disentangle somecompeting disturbing effects acting as sources of systematic bias. Nonetheless,for some reasons unknown to the present author, $\eta$ has never been used sofar by astronomers in actual data reductions. This note aims to raise interestin the community about the possible practical use of such an orbital elementor, at least, to induce experts in astronomical data processing to explicitlymake clear if it is not possible to use $\eta$ for testing gravitational modelsand, in this case, why.

Why the mean anomaly at epoch is not used in tests of non-Newtonian gravity?