Keplerian and Newtonian physics tell us that the rotational velocities of planets must decrease with any increase in radial distance from the sun. The same theoretical assumption is made for the rotational velocities of all visible mass including stars around spiral galaxy cores. As spiral galaxy brightness profiles diminish in luminosity from core to disc edge the assumptions to date are that because the observed rotation curves of galaxies are flat, this is not consistent with the distribution of visible mass in spiral galaxy discs. And that either new physics or dark matter halos must be invoked to explain why the observed flat rotation curves do not match the rapidly and exponentially diminishing brightness of the galaxy’s surface brightness profile as the radius increases. Here in this paper a new rotation curve calculation is made which shows that the surface brightness profiles of visible mass in galaxies can be made to favourably fit the observed flat galaxy rotation curves without having to invoke any new physics or dark matter.