A system and method to determine angular movement of an arbor integral with a crown of a watch, the arbor being rotatable on itself in a longitudinal direction. The system includes a rotating reflector mounted on the arbor, and two emitter/detector pairs disposed on either side of the reflector, each including a light source for illuminating the reflector, and a light detector for receiving the light reflected on the reflector and for generating an electrical signal representative of the reflected light. A processor processes the electrical signals, and determines a parameter relating to the angular movement of the arbor. The rotating reflector is a cylinder of revolution and light absorption points are made on the circumference of an external cylindrical surface. When the reflector rotates in a regular manner and in the same direction of rotation, the representative electrical signal generated by each detector has a sinusoidal shape.