I have a question about the gravitational model used to describe tides: Is it correct to simply add the tide potentials respectively due to earth and sun as most people do? (see for instance a good example of this approach in Eq 9 of: http://arxiv.org/PS_cache/physics/pdf/0701/0701301v1.pdf). The superposition principle, at first, almost convinced me not to go further... when I came across this 1976 paper: http://adsabs.harvard.edu/full/1977SvAL....3...96A, which is then contradicted in 1981 by: http://articles.adsabs.harvard.edu//full/1981SvAL....7..281S/0000282.000.html. Avsyuk's reasoning is appealing: to calculate the acceleration a0 of M0 in the inertial coordinate system of the three bodies he adds the acceleration a1 of M0 in the mobile system of the earth-moon barycenter to the acceleration a2 of this barycenter with respect to the earth-moon-sun reference system. Sitnik and Khlystov have the same expression for a2 (Eq. 6) but they differ for a1. I have a hard time to understand their second term in (8). What is M0 acceleration in the mobile refrerence frame of the earth-moon barycenter? Is it Sitnik and Khlystov's Eq.8 or F3/M0 as Avsyuk wants or, as I would prefer, (F1+F3)/M0? Does someone would care to ponder a bit on these questions? Thanks!