They clearly had a concept of similar triangles, and a concept similar to our notion of “slope” or “tangent” (apparently they actually used the reciprocal of sl

They clearly had a concept of similar triangles, and a concept similar to our notion of “slope” or “tangent” (apparently they actually used the reciprocal of slope, i.e. length per unit depth). However, they may not have had the concept as of ~4000 years ago of “angle measure” per se. You’d have to ask an expert in the history of astronomy when that concept was first developed semi-formally; my understanding is that it was 1000–1500 years later by subsequent Mesopotamian astronomers. Angle measure (another word for circular arclength corresponding to a particular rotation) is actually quite a tricky thing to get right. It’s the logarithm of the rotation (a pure bivector quantity) with the orientation stripped away – if we call our rotation R oriented in the plane of the unit bivector i, it’s θ = log(R)/i. To go from an angle measure back to a “complex number” (scalar + bivector) with which you can do anything useful, you need to put the orientation back in and then exponentiate it: R = exp(iθ). In dimensions higher than 2, you need to use rotation operators via a kind of sandwich product, because composition of rotations is not commutative. This is not an obvious concept, didn’t really get figured out properly until the late 19th century, and still isn’t taught very clearly to students today.