  The transformation of the partial fractional derivatives under spatial
rotation in $R^2$ are derived for the Riemann-Liouville and Caputo definitions.
These transformation properties link the observation of physical quantities,
expressed through fractional derivatives, with respect to different coordinate
systems (observers). It is the hope that such understanding could shed light on
the physical interpretation of fractional derivatives. Also it is necessary to
able to construct interaction terms that are invariant with respect to
equivalent observers.
