  In physics we find such a fiction in the fact that masses of undeniable extension, e.g., the sun and the earth, in connection with the derivation of certain basic concepts of mechanics and the calculation of their reciprocal attraction are reduced to points or concentrated into points (gravitational points) in order, by means of this fiction, to facilitate the presentation of the more composite phenomena. Such a neglect of elements is especially resorted to where a very small factor is assumed to be zero. (PAO  16)