The first term is the gravitational acceleration due to M at the center of the reference body m , i.e., at the point where Δ Δ r is zero. This term does not affect the observed acceleration of particles on the surface of m because with respect to M, m (and everything on its surface) is in free fall. When the force on the far particle is subtracted from the force on the near particle, this first term cancels, as do all other even-order terms. The remaining (residual) terms represent the difference mentioned above and are tidal force (acceleration) terms. When ∆ r is small compared to R, the terms after the first residual term are very small and can be neglected, giving the approximate tidal acceleration a → → t , axial for the distances ∆ r considered, along the axis joining the centers of m and M :