A class of diffeomorphism invariant, physical observables (so-called astrometric observables) is introduced. A particularly simple example, the time delay, which expresses the difference between two initially synchronized proper time clocks in relative inertial motion, is analyzed in detail. It is found to satisfy some interesting inequalities related to the causal structure of classical Lorentzian space times. Some potential applications of astrometric observables in quantum gravity are discussed.