  Light scalars can in principle couple to both bulk matter and fermion spin,
with hierarchically disparate strengths. Storage ring measurements of fermion
electromagnetic moments via spin precession can be sensitive to such a force,
sourced by the Earth. We discuss how this force could lead to a deviation of
the measured muon anomalous magnetic moment, $g-2$, from the Standard Model
prediction. Due to its different parameters, the proposed JPARC muon $g-2$
experiment can provide a direct test of our hypothesis. A future search for the
proton electric dipole moment can have good sensitivity for the coupling of the
assumed scalar to nucleon spin. We also argue that supernova constraints on
axion-muon coupling may not be applicable in our framework.
