  Femtoscopy of non-central heavy ion collisions provides access to information
on the geometry of the effective pion-emitting source. In particular, the
source shape can be studied by measuring femtoscopic radii as a function of the
pion emission angle relative to the collision symmetry planes. We present the
results of azimuthally differential femtoscopy of Pb-Pb collisions at
$\sqrt{s_{NN}}=2.76$ TeV at the LHC relative to the second harmonic event
plane. We observe a clear oscillation of the extracted radii as a function of
the emission angle. We find that $R_{side}$ and $R_{out}$ oscillate out of
phase for all centralities and pion transverse momenta. The relative amplitude
of $R_{side}$ oscillation decreases in more central collisions, but remains
positive, which indicates that the source remains out-of-plane extended
qualitatively similar to what was observed at RHIC energies. We compare our
results to existing hydrodynamical and transport model calculations.
