The Occultation of 28 Sgr by Saturn: Saturn Pole Position and Astrometry

We determine the geometry of the 3 July 1989 occultation of 28 Sgr by Saturn and use the results to derive Saturn's pole as defined by the ring plane. The analysis makes use of high-precision timings of 12 circular edges in the C-ring (radii = 84750 to 90615 km), three features in the Cassini division, the edges of the Encke gap, and the outer edge of the Keeler gap. Our group of observing sites includes one southern hemisphere station (CTIO) and five northern hemisphere stations (three Tucson-area stations, the UK Infrared Telescope in Hawaii, and San Pedro Mártir in Baja California), which provides a terrestrial chord separation ∼10,000 km. Assuming ring radii obtained from an analysis of Voyager radio- and stellar-occultation data (P. D. Nicholson, M. L. Cooke, and E. Pellon, 1990, Astron. J.100, 1339-1362), we use edge timings to solve for the position angle and opening angle of the apparent ring ellipses. The star 28 Sgr appears to be well represented as a uniformly illuminated disk with an apparent radius of 9.0 ± 0.3 km projected at Saturn at the wavelengths of observation (λ ∼ 3 μm). The internal consistency of the data set and redundancy of stations indicates that the relative positional error of a given sharp ring edge timing is at the level of 2 km or smaller; the absolute error is roughly 5 km. Our derived pole position, α_P = 40.599° ± 0.020°, δ_P = 83.536° ± 0.005°, is consistent with the pole and ring radius scale deduced from Voyager stellar and radio occultation observations (P. D. Nicholson, M. L. Cooke, and E. Pelton, 1990, Astron. J.100, 1339-1362) and the solution found in the accompanying paper (French et al., 1993, this issue).