The transit time of a photon or electromagnetic wave between two points in space is a measurement that is used to determine the orbits of the planets and spacecraft for the purposes of navigation and science. Both the navigation of a spacecraft and science experiments, particularly associated with General Relativity, require precise measurements of the transit time. Since the Deep Space tracking stations can measure times to within 0.1 ns or about 3 cm, it is necessary to model the transit time to this accuracy. In this paper, a formula is developed for the total one-way transit time of a photon in the coordinate time of General Relativity. A bending term is identified that contributes about 10 ns to the transit time of a photon that grazes the Sun and is not generally included in -other formula in the literature. The results are verified by performing numerical integration of the equations of motion for a photon. A numerical photon ephemeris is computed and compared with the analytic formula.
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