A system for measuring a variation in a distance of an axis of rotation of an object relative to a point in a plane perpendicular to this axis comprises at least two sensors for measuring a value representing an angular position of said object about said axis, at least a first of said sensors, whose position is fixed relative to said point, comprising a magnetometer suitable for measuring the magnetic field generated by said object whose magnetization varies around its circumference; and means for determining said variation of distance on the basis of a variation in an angular deviation between said sensors relative to the axis, from one revolution of said object to the next.