  We study the spectrum $\{\lambda_j(m)\}_{j=1}^{\infty}$ of a timelike Killing
vector field $Z$ acting as a differential operator $D_Z$ on the Hilbert space
of solutions of the massive Klein-Gordon equation $(\Box_g + m^2) u = 0$ on a
globally hyperbolic stationary spacetime $(M, g)$ with compact Cauchy
hypersurface. The inverse mass $m^{-1}$ is formally like the Planck constant in
a Schr\"odinger equation, and we give Weyl asymptotics as $m \to \infty$ for
the number $$N_{\nu, C}(m)= \# \{j \mid \frac{\lambda_j(m)}{m} \in [\nu -
\frac{C}{m}, \nu + \frac{C}{m} ]\}$$ for a given $C > 0$. The semi-classical
mass asymptotics are governed by the dynamics of the Killing flow $e^{tZ} $ on
the hypersurface in the space of mass $1$ geodesics $\gamma$ where $\langle
\dot{\gamma}, Z \rangle= \nu$.
