On angular measures in axiomatic Euclidean planar geometry

Abstract

We address the issue of angular measure, which is a contested issue for the International System of Units (SI).We provide a mathematically rigorous and axiomatic presentation of angular measure that leads to the traditional way ofmeasuring a plane angle subtended by a circular arc as the length of the arc divided by the radius of the arc, a scalar quantity.We distinguish between theangular magnitude, defined in terms of congruence classes of angles, and the (numerical)angularmeasurethat can be assigned to each congruence class in such a way that, e.g., the right angle has the numerical valueπ2.We argue that angles are intrinsically different from lengths, as there are angles of special significance (such as the rightangle, or the straight angle), while there is no distinguished length in Euclidean geometry. This is further underlined by theobservation that, while units such as the metre and kilogram have been refined over time due to advances in metrology, nosuch refinement of the radian is conceivable. It is a mathematically defined unit, set in stone for eternity. We conclude thatangular measures are numbers, and the current definition in SI should remain unaltered.