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P. D. Nicholson (Cornell), R. G. French (Wellesley College)
The predicted precession rate of Saturn's pole, due principally to the indirect solar torque on Titan, is -0.738'' yr-1, corresponding to a period of 1.76~My (French et al. [1993] Icarus 103, 163). Analysis of subsets of Voyager, ground-based and HST occultation data have led to experimental determinations of -0.63''±.23'' (French et al. 1993) and -0.41''±.19'' (A. Bosh [1994] Ph.D. thesis, MIT). Combining this information with the observed time of ring plane crossing on 22 May 1995, Bosh et al. [1997] (Icarus 129, 555) derived an annual rate of -0.52''±.07'', consistent with the occultation results but significantly less than the predicted value. Precise observations of the timing of Saturn's historical ring plane crossings can provide an independent method for determining the planet's axial precession rate. The first such estimate was made by F. Bessel in 1833. We describe here an analysis of 22 published times of ring plane crossing, extending over a period of 280~yr. Apart from those made with HST in May and August 1995 (Bosh et al.; Nicholson et al. 1996 [Science 272, 509]), the most important observations were made at Pic du Midi in 1966 (Dollfus 1979 A. & A. 75, 204), and at Johannesberg and Yerkes in 1907/08 (Innes 1908 MNRAS 68, 32); Barnard 1908 Ibid 68, 346). A weighted least-squares fit to all of the available data yields a precession rate for the ring plane of -0.51±.14''~yr-1, or 0.69±.19 times the predicted rate. Our fit is also consistent with the very close approach of the Earth to the ring plane observed in December 1936. It appears that the low precession rate is primarily a consequence of Titan's proper inclination of 0.32\circ, which leads to a slow variation in the torque exerted on Saturn with a period of ~00~yr, Titan's nodal regression period (Nicholson et al. [1999] BAAS 31, 1140).
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.