The observation of superluminal motion in extragalactic radio jets has spurred the notion that these jets are moving relativistically. Doppler boosting due to relativistic motion has also been used to explain the observed one-sidedness of these jets, and the high surface brightnesses encountered (Kellermann and Pauliny-Toth 1981, Zensus and Pearson 1990, Zensus and Kellermann 1994, Cawthorne 1991). The symmetric twin-relativistic jet model (STRJM) is an integral part of theories concerning the unification of powerful radio galaxies and quasars (e.g., Barthel et al. 1989, Antonucci 1993). Cygnus A provides an important test, since the jets in Cygnus A are thought to be close to the sky plane, as deduced from the large-scale morphology of the source (Hargrave and Ryle 1974, Scheuer 1983). The predictions are that for such a source the jet motion will be trans-luminal and the ratio of flux density in the jet to that in the counterjet will not be extreme.
The first VLBI images of Cygnus A revealed a pc-scale core source with a
slight elongation along the axis of the kpc-scale source
(Kellermann et al. 1981,
1975).
This elongation was resolved into a core-jet by
Linfield (1981)
of 
4 mas
length. Further VLBI imaging
revealed a `knotty' jet extending 20 mas from the core source at the
same position angle as the kpc-scale jet, and proper motion for
one of the jet components was measured
(Carilli et al. 1989b,
1994a).
The most recent VLBI studies of Cygnus A are by
Sorathia et al. (1996)
and Krichbaum et
al. (1996).
Both these studies detect a counterjet in
Cygnus A. Sorathia et al. use three epochs of observations to
determine the apparent velocity,
A (in
units of c), of the outermost jet component (J4) to be:
A = 0.55
± 15 h-1,
and they find a value for the ratio of the integrated flux density in
the jet to that in the counterjet, R, of: R = 5 ± 2 at 5 GHz. Using
these two parameters
(A and
R), they apply the equations for the STRJM
(Scheuer and Readhead
1979)
to constrain the jet angle relative to
our line of sight, 
, and to
constrain the true jet velocity,
:
and
For
h = 1/2 they find 1.0
The higher resolution observations of
Krichbaum et al. (1996)
present a
somewhat more complicated picture. The complexity and temporal
variability of the sub-pc-scale structure makes core idenfication
difficult. Making a nominal core identification with the most inverted
spectrum component, Krichbaum et al. find evidence that the inner jet
components may have a range in apparent velocities
(
A =
sin /
(1 -
cos)
cos) /
(1 -
cos))2-
0.6 and 85°
50°. Hence, the jet in Cygnus A must be at an intermediate
angle relative to our line of sight, and moving at a mildly relativistic
velocity.
A between
0.2 and 0.8 h-1). This would be similar to the well studied case
of the jet in M 87 for which knot velocities ranging from sub- to
superluminal have been observed
(Biretta et al. 1995).
Such an
observation suggests that the apparent velocities represent a pattern
speed, eg. of surface waves on the jet, and not the true jet fluid
velocity, thereby invalidating the standard STRJM analysis.