Astrometry of the main satellites of Uranus: 18 years of observations

Context. We contribute to developing dynamical models of the motions of Uranus’ main satellites. Aims. We determine accurate positions of the main satellites of Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon. Positions of Uranus, as derived from those of these satellites, are also determined. The observational period spans from 1992 to 2011. All runs were made at the Pico dos Dias Observatory, Brazil. Methods. We used the software called Platform for Reduction of Astronomical Images Automatically (PRAIA) to perform a digital coronography to minimise the influence of the scattered light of Uranus on the astrometric measurements and to determine accurate positions of the main satellites. The positions of Uranus were then indirectly determined by computing the mean differences between the observed and ephemeris positions of these satellites. A series of numerical filters was applied to filter out spurious data. These filters are mostly based on (a) the comparison between the positions of Oberon with those of the other satellites and on (b) the offsets as given by the differences between the observed and ephemeris positions of all satellites. Results. We have, for the overall offsets of the five satellites, −29 mas (±63 mas) in right ascension and −27 mas (±46 mas) in declination. For the overall difference between the offsets of Oberon and those of the other satellites, we have +3 mas (±30 mas) in right ascension and −2 mas (±28 mas) in declination. Ephemeris positions for the satellites were determined from DE432+ura111. Comparisons using other modern ephemerides for the solar system – INPOP13c – and for the motion of the satellites – NOE-7-2013 – were also made. They confirm that the largest contribution to the offsets we find comes from the motion of the barycenter of the Uranus system around the barycenter of the solar system, as given by the planetary ephemerides. For the period from 1992 to 2011, our final catalogues contain 584 observed positions of Miranda, 1710 of Ariel, 1987 of Umbriel, 2588 of Titania, 2928 of Oberon, and 3516 of Uranus