Abstract. At the oppositions of 1995-1997, a total of 122 CCD frames were taken on the 1.56 m astrometric telescope at the Sheshan station, yielding 864 positions of the major Uranian satellites. The calibration of the images was carried out using a least-squares iterative program by fitting to the well-known orbits of the brighter moons of Uranus, based on the modern theory GUST86 and an ephemeris produced by numerical integration. The residuals lay between 0. 03 and 0. 05 for each of the inter-satellite positions, except for the innermost and faintest satellite Miranda, whose residuals exceeded 0. 08 due to the proximity of Uranus. No significant systematic errors were found when using satellites themselves for determining calibration parameters. The largest residual in the comparison between GUST86 and the numerical integration was about 0. 01.

We study numerically the three-dimensional periodic motion of a massless particle in an annular configuration of N bodies. This ring model can approximate the dynamics of celestial systems like clusters and planetary rings. After calculating numerically the families of x-symmetric periodic orbits in the planar case for a certain N, we investigate the families' vertical stability and locate those members that present critical instability in the eigenvalues corresponding to the z-axis. These orbits are the points of bifurcation of the planar families of periodic orbits to three-dimensional families. By means of differentially correcting the vertically critical orbits we extend our numerical investigation in the three-dimensional space and locate several families of three-dimensional periodic orbits symmetric to the xz-plane. The evolution of the families and the significant features of their members are subsequently presented.

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We study the bifurcations of three-dimensional periodic motions from two-dimensional orbits of small particles in the neighborhood of a system that consists of three major bodies being always in syzygy. We assume that two of them have equal masses and are located at equal distances from the third body which has a different mass. All or some of the primaries are radiation sources and therefore apart from gravitational forces, we also consider forces that result from radiation. We apply the method of vertical critical stability in order to find the families of three-dimensional periodic orbits that bifurcate from families of planar periodic motions. Consequently, we study the parametric variations of the bifurcation points.

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