  This article proposes a comprehensive analysis of light propagation in an
anisotropic and spatially homogeneous Bianchi I universe. After recalling that
null geodesics are easily determined in such a spacetime, we derive the
expressions of the redshift and direction drifts of light sources; by solving
analytically the Sachs equation, we then obtain an explicit expression of the
Jacobi matrix describing the propagation of narrow light beams. As a byproduct,
we recover the old formula by Saunders for the angular diameter distance in a
Bianchi I spacetime, but our derivation goes further since it also provides the
optical shear and rotation. These results pave the way to the analysis of both
supernovae data and weak lensing by the large-scale structure in Bianchi
universes.
