  According to Mach's principle inertia has its reason in the presence of all
masses in the universe. Despite there is a lot of sympathy for this plausible
idea, only a few quantitative frameworks have been proposed to test it. In this
paper a tentative theory is given which is based on Mach's critisism on
Newton's rotating bucket. Taking this criticism seriously, one is led to the
hypothesis that the rotation of our galaxy is the reason for gravitation.
Concretely, a functional dependence of the gravitational constant on the size,
mass and angular momentum of the milky way is proposed that leads to a spatial,
but not to a temporal variation of G. Since Newton's inverse-square law is
modified, flat rotation curves of galaxies can be explained that usually need
the postulate of dark matter. While the consequences for stellar evolution are
discussed briefly, a couple of further observational coincidences are noted and
possible experimental tests are proposed.
