  Context. 1E 1740.7-2942 is believed to be one of the two prototypical
microquasars towards the Galactic center region whose X-ray states strongly
resemble those of Cygnus X-1. Yet, the bipolar radio jets of 1E 1740.7-2942 are
very reminiscent of a radio galaxy. The true nature of the object has thus
remained an open question for nearly a quarter of a century. Aims. Our main
goal here is to confirm the Galactic membership of 1E 1740.7-2942 by searching
for morphological changes of its extended radio jets in human timescales. This
work was triggered as a result of recent positive detection of fast structural
changes in the large-scale jets of its 'twin' GRS 1758-258. Methods. We carried
out an in-depth exploration of the VLA public archives and fully recalibrated
all 1E 1740.7-2942 extended data sets in the C configuration of the array. We
obtained and analyzed matching beam radio maps for five epochs, covering years
1992, 1993, 1994, 1997 and 2000, with an angular resolution of a few
arcseconds. Results. We clearly detected structural changes in the arcminute
jets of 1E 1740.7-2942 on timescales of roughly a year, which set a firm
distance upper limit of 12 kpc. Moreover, a simple precessing model was
simultaneously fitted to the five observing epochs available. The observed
changes in the jet flow are strongly suggestive of a precession period of ~1.3
years. Conclusions. The fitting of the precession model to the data yields a
distance of ~5 kpc. This value, and the observed changes, rule out any
remaining doubts about the 1E 1740.7-2942 Galactic nature. To our knowledge,
this microquasar is the second whose jet precession ephemeris become available
after SS433. This kind of information is relevant to the physics of compact
objects, since the genesis of the precession phenomenon occurs very close to
the interplay region between the accretion disk and the compact object in the
system.
