  Er_2Ti_2O_7 has been suggested to be a realization of the frustrated < 111 >
XY pyrochlore lattice antiferromagnet, for which theory predicts
fluctuation-induced symmetry breaking in a highly degenerate ground state
manifold. We present a theoretical analysis of the classical model compared to
neutron scattering experiments on the real material, both below and above T_N =
1.173(2) K. The model correctly predicts the ordered magnetic structure,
suggesting that the real system has order stabilised by zero-point quantum
fluctuations that can be modelled by classical spin wave theory. However, the
model fails to describe the excitations of the system, which show unusual
features.
