Left: Outline of the limb‐brightened head of the Guitar Nebula in 1994 (filled circles) and 2001 (open circles), obtained by eye. The images are aligned using eight bright stars in the PC image at each epoch: to demonstrate the image registration, two stars in the PC image section displayed in Fig. 1 are marked with crosses in 1994 and open circles in 2001. Right: Slices through the best‐fit point on the χ² surface for the model fit procedure on 1994 data (filled circles) and 2001 data (open circles), showing the relative change in χ² as a function of the scale factor ﹩S=D^{-2}_{\mathrm{kpc}\,}n^{-1/ 2}_{A}﹩. The implied change in ambient density at the nebula tip is ﹩n_{A}( 2001) / n_{A}( 1994) \approx 0.7﹩.