  Our knowledge of dynamical black holes suffers from a lack of observational
insight. In an analogue model of gravity, we can design a longitudinally
symmetric dynamical acoustic black hole with a moving horizon. In this
symmetric spacetime, the marginally outer trapped surface and the so called
evolving horizon are degenerate. Interestingly, there are many ways of
assigning a surface gravity to the horizon in the absence of time translation
invariance. Here, we present two of them that are distinguished by whether they
are defined only locally or take into account the global properties of
spacetime outside the black hole. It is expected that a dynamical black hole
would emit spontaneous thermal radiation and its temperature would be
proportional to a surface gravity of the moving horizon but there is no
consensus on which of the surface gravities would give the Hawking temperature.
We propose that a non-stationary analogue spacetime when realised
experimentally can possibly help resolve these mysteries and provide the first
observational signatures of dynamical Hawking radiation.
