The Spatial Geometry of a Uniformly Rotating Reference Frame JS Model explores the spatial geometry of the relativistic and the Newtonian reference frames. The left window depicts a Newtonian world and the right, a relativistic. In both cases, there has been drawn a circle whose radius R can be controlled by the user. The user can drag and transfer a vector parallel to itself along the boundary of the circle. In the relativistic world, the rest plane is not Euclidean and the parallel displacing vector when returns at its initial position has a direction which is in general different from the original. By using the theoretical model (see the attached pdf-file), and the measured angles, the user can calculate the angular velocity of the rotating frame.  Furthermore, the relativistic observer measures the length L of the boundary of the circle and its diameter, and he finds out that their ratio is different of pi; it depends on the angular velocity of the rotating frame  and the radius of the circle.