In a photo each pixel represents an angle.
In an image of two objects with known angle (e.g. two stars), one could count the number of pixels between them and determine the ratio pixels/angle degree of that camera for all other purposes.
I try to ignore the true ratio and just use pixel as an angular unit. However, trigonomectric functions like sine and cosine fail to work then, it seems. Why is that? I mean, what if I had a camera with very many pixels per angular degree, and one with extremely few, why should it matter? Why can't I simply define that one pixel represents one degree? Or ten?