The angular expansion rate of the X-ray remnant of the supernova (SN)
observed by Tycho Brahe in AD 1572 has been determined using
observations by ROSAT. From two High Resolution Imager (HRI)
pointings separated in time by 4.5 yr, I determine a mean expansion
rate for the blast wave of
yr
, which is larger than published results from optical and radio
observations. The X-ray expansion is consistent with the dynamical
evolution expected from a young ejecta-dominated remnant which has
experienced only modest deceleration. I find evidence for differential
expansion in that the interior of the remnant appears to be expanding
at a somewhat slower rate. The expansion rate varies significantly
with azimuth as well. In particular, I find that two small knots in
the southeastern part of Tycho are moving more rapidly than any other
feature in the X-ray image. Previous work has established that these
knots are clumps of highly enriched stellar ejecta with very different
abundance distributions. This is the first evidence for ejecta
``bullets'' in the remnant of a Type Ia SN.