On angular measures in axiomatic Euclidean planar geometry.

We address the issue of angle measurements which is a contested issue for the International System of Units (SI). We provide a mathematically rigorous and axiomatic presentation of angle measurements that leads to the traditional way of measuring a plane angle subtended by a circular arc as the length of the arc divided by the radius of the arc, a scalar quantity. This abstract approach has the advantage that it is independent of the assumptions of Cartesian plane geometry. We distinguish between the geometric angle, defined in terms of congruence classes of angles, and the (numerical) angular measure that can be assigned to each congruence class in such a way that, e.g., the right angle has the numerical value {\pi}/2 . We argue that angles are intrinsically different from lengths, as there are angles of special significance (such as the right angle, or the straight angle), while there is no distinguished length in Euclidean geometry. This is further underlined by the observation that, while units such as the metre and kilogram have been refined over time due to advances in metrology, no such refinement of the radian is conceivable. It is a mathematically constructed unit, set in stone for eternity. We conclude that angle measurements are dimensionless, and the current definition in SI should remain unaltered.