Abstract
The time history of BL Lacertae has shown clear evidence of changes in jet orientation both in the plane of the sky and in the angle to the line of sight (see Figure 1). Models based on transverse shocks in a relativistic flow quantitatively fit the polarization and flux density data well and permit one to determine parameters of the flow such as the bulk Lorentz factor and the angle of the flow to the line of sight (Hughes, Aller and Aller 1989). The orientation of the jet flow to the line of sight changed by approximately 6° between the early 1980 bursts and one in 1991. There have been comparable changes in the orientation of the jet on the plane of the sky. Such changes in jet orientation may be due to a helical flow pattern arising from precession or instability.
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