Phase-referenced VLBI observations of supernova 1993J at 24 epochs , from 50 days after shock breakout to the present , allowed us to determine the coordinates of the explosion center relative to the quasi-stationary core of the host galaxy M81 with an accuracy of 45 \mu as , and to determine the nominal proper motion of the geometric center of the radio shell with an accuracy of 9 \mu as yr ^ { -1 } .
The uncertainties correspond to 160 AU for the position and 160 km s ^ { -1 } for the proper motion at the distance of the source of 3.63 Mpc .
After correcting for the expected galactic proper motion of the supernova around the core of M81 using HI rotation curves , we obtain a peculiar proper motion of the radio shell center of only 320 \pm 160 km s ^ { -1 } to the south , which limits any possible one-sided expansion of the shell .
We also find that the shell is highly circular , the outer contours in fact being circular to within 3 % .
Combining our proper motion values with the degree of circular symmetry , we find that the expansion of the shockfront from the explosion center is isotropic to within 5.5 % in the plane of the sky .
This is a more fundamental result on isotropic expansion than can be derived from the circularity of the images alone .
The brightness of the radio shell , however , varies along the ridge and systematically changes with time .
The degree of isotropy in the expansion of the shockfront contrasts with the asymmetries and polarization found in optical spectral lines .
Asymmetric density distributions in the ejecta or more likely in the circumstellar medium , are favored to reconcile the radio and optical results .
We see no sign of any disk-like density distribution of the circumstellar material , with the average axis ratio of the radio shell of SN1993J being less than 1.04 .