The gaseous remnants of historical supernova explosions provide
astronomers with a valuable class of objects for investigating the
dynamical evolution of supernova remnants (SNRs). For these remnants
one knows the average expansion rate and thus a determination of the
current expansion rate yields an indication of the remnant's
deceleration. However the expected motions are small,
;SPMlt;1 
yr 
, and thus significant measurements of
current expansion rates in SNRs require high angular resolution, long
time baselines, and deep observations.
Tycho's SNR is an ideal target for such studies. Indeed, the expansion
of Tycho's SNR has been measured previously in both the optical
(Kamper & van den Bergh 1978) and radio bands (Tan & Gull 1985). The
observed proper motion of the optical filaments indicate expansion
rates ranging from 0.18 yr to
0.28 yr . Since the amount of material probed
by the optical emission is a tiny fraction of the total mass of
Tycho's SNR and filaments appear only toward the north and east near
the remnant's periphery, observations in the optical band are unable
to provide a comprehensive picture of the expansion of the remnant.
Radio data reveal a nearly circular, limb-brightened, clumpy shell of
emission. The overall expansion of the entire radio remnant has been
determined to be 
% yr , although azimuthal
sectors of the remnant are observed to expand at rates varying from
0.05 % yr to 0.15 % yr or more. Again, the
radio band probes material different from that in the X-ray or
optical: in this case regions of compressed magnetic field and
relativistic electron acceleration.