  Long-time atom interferometry is instrumental to various high-precision
measurements of fundamental physical properties, including tests of the
equivalence principle. Due to rotations and gravity gradients, the classical
trajectories characterizing the motion of the wave packets for the two branches
of the interferometer do not close in phase space, an effect which increases
significantly with the interferometer time. The relative displacement between
the interfering wave packets in such open interferometers leads to a fringe
pattern in the density profile at each exit port and a loss of contrast in the
oscillations of the integrated particle number as a function of the phase
shift. Paying particular attention to gravity gradients, we present a simple
mitigation strategy involving small changes in the timing of the laser pulses
which is very easy to implement. A useful representation-free description of
the state evolution in an atom interferometer is introduced and employed to
analyze the loss of contrast and mitigation strategy in the general case. (As a
by-product, a remarkably compact derivation of the phase-shift in a general
light-pulse atom interferometer is provided.) Furthermore, exact results are
obtained for (pure and mixed) Gaussian states which allow a simple
interpretation in terms of the alignment of Wigner functions in phase-space.
Analytical results are also obtained for expanding Bose-Einstein condensates
within the time-dependent Thomas-Fermi approximation. Finally, a combined
strategy for rotations and nonaligned gravity gradients is considered as well.
