  In this paper, it is shown, using a geometrical approach, the isotropy of the
velocity of light measured in a rotating frame in Minkowski space-time, and it
is verified that this result is compatible with the Sagnac effect. Furthermore,
we find that this problem can be reduced to the solution of geodesic triangles
in a Minkowskian cylinder. A relationship between the problems established on
the cylinder and on the Minkowskian plane is obtained through a local isometry.
