Lengths on rotating platforms

The paper treats the issue of the length of a rotating circumference as see n from on board the moving disk and from an inertial reference frame. It is shown that, properly defining a measuring process, the result is in both c ases 2 pi R thus dissolving the Ehrenfest paradox. The same holds good when considering that, for the rotating observer, the perceived radius coincide s with the curvature radius of a space-time helix and a complete round trip corresponds to an angle which differs from the one seen by the inertial ob server. The apparent contradiction with the Lorentz contraction is discusse d.