We present the deepest yet radio image of the Galactic jet source , SS 433 , which reveals over two full precession cycles ( > 2 \times 163 days ) of the jet axis .
Systematic and identifiable deviations from the traditional kinematical model for the jets are found : variations in jet speed , lasting for as long as tens of days , are necessary to match the detailed structure of each jet .
It is remarkable that these variations are equal and opposite , matching the two jets simultaneously .
This explains certain features of the correlated redshift residuals found in fits to the kinematic model of SS 433 reported in the literature .
Asymmetries in the image caused by light travel time enabled us to measure the jet speeds of particular points to be within a range from 0.24 c to 0.28 c , consistent with , yet determined independently from , the speeds derived from the famous moving optical emission lines .
Taken together with the angular periodicity of the zigzag/corkscrew structure projected on the plane of the sky ( produced by the precession of the jet axis ) , these measurements determine beyond all reasonable doubt the distance to SS 433 to be 5.5 \pm 0.2 kpc , significantly different from the distance most recently inferred using neutral hydrogen measurements together with the current rotation model for the Galaxy .