  We approach the discretisation of Maxwell's equations directly in space-time
without making any non-relativistic assumptions with the particular focus on
simulations in rotating reference frames. As a research example we study
Sagnac's effect in a rotating ring resonator. After the discretisation, we
express the numerical scheme in a form resembling 3D FIT with leapfrog. We
compare the stability and convergence properties of two 4D approaches, namely
FIT and FEM, both using Whitney interpolation.
