Imagine that Earth rotated about its axis once per month and that the Moon moved in a circular orbit above Earth’s equator. Under these tidally locked conditions, the Moon and the resulting tidal bulge in Earth’s oceans would remain fixed in an Earth-based reference system. Give an estimate of the tidal displacement in this situation by computing the gravitational potential along a line passing through the centers of the Moon and Earth and comparing to the potential at a second point also on Earth’s equator but perpendicular to this line. Expand the potentials in R/r and Δ/R (in the notation of eq. (31.25)) and discard the term linear in R that is associated with the Moon’s gravitational force acting on the full Earth.