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P.D. Nicholson (Cornell), R.G. French (Wellesley), A.S. Bosh (Lowell)
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 (28 Sgr, July 1989) and 1991 HST occultation data led to best-fit annual rates of -0.63''±.23'' (French et al. 1993) and -0.41''±.19'' (Bosh [1994] Ph.D. thesis, MIT). The latter is only 55% of the predicted rate. Precise observations of Saturn's ring plane crossings provide an independent method for determining the planet's axial precession rate. Combining the known pole position at the time of the Voyager encounters 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 and again significantly less than the predicted value.
An analysis of 22 reported times of ring plane crossing, extending over a period of 280~yr, leads to a very similar estimate of the pole precession rate of -0.51''±.14''~yr-1, which is completely independent of the occultation data (Nicholson & French [1997] BAAS 29, 1097). The most important observations are those made with HST in May and August 1995 (Bosh et al. 1997; Nicholson et al. 1996 [ Science 272, 509]), 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). The time used for the 10 August 1995 crossing is that reported for the west ansa (20:20±8~min UT), as the east ansa was partially obscured by the F Ring at this time. It now 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. At the present time, the torque is close to its minimum value. The nutation model of Vienne & Duriez [1992] (A & A 257, 331) in fact predicts an average precession rate of -0.50''~yr-1 over the last 100~yr, 68% of the secular rate and in excellent agreement with the observations.