  Long-wavelength matter inhomogeneities contain cleaner information on the
nature of primordial perturbations as well as the physics of the early
universe. The large-scale coherent overdensity and tidal force, not directly
observable for a finite-volume galaxy survey, are both related to the Hessian
of large-scale gravitational potential and therefore of equal importance. We
show that the coherent tidal force causes a homogeneous anisotropic distortion
of the observed distribution of galaxies in all three directions, perpendicular
and parallel to the line-of-sight direction. This effect mimics the
redshift-space distortion signal of galaxy peculiar velocities, as well as a
distortion by the Alcock-Paczynski effect. We quantify its impact on the
redshift-space power spectrum to the leading order, and discuss its importance
for the ongoing and upcoming galaxy surveys.
