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\def\tc{3C\kern0.1em 47} Dual-epoch VLBI observations of \tc\ at 10.7\,GHz reveal superluminal motion with $\beta_{\rm app}=(3.7^{+0.9}_{-0.7})\,h^{-1} \,c$. Following the standard beaming model, this implies that the deprojected (intrinsic) total linear size of \tc\ is significantly larger than its observed extent. This is surprising, since, in observed extent, \tc\ is already the largest quasar in our complete, orientation unbiased quasar sample. The beaming implied by the observed superluminal motion also means that it does not easily fit the observed distribution of core-to-lobe flux density ratios ($R$) in the sample.
This is the first somewhat surprising superluminal motion out of the seven sources in the sample for which a velocity or upper limit is now known. It is possible that \tc\ has a rather higher Lorentz factor than most of the other double-lobed quasars observed to date. However, the study of this sample is being undertaken precisely because the standard model presents a clear prediction of the velocity distribution, and if more velocities would show a definite deviation from this distribution, this would have very significant consequences. The program can now be pursued vigorously with the newly available VLBA. Once more measurements are available, a careful assessment of the distribution of the intrinsic source properties will be a necessary part of the study of the range of Lorentz factors and of a possible cutoff in jet orientation angles.
