Occultation of beta Scorpii by Jupiter and Io. III. Astrometry

The timings of the occultation events of 13-14 May 1971 are used to derive a value for the equatorial radius and oblateness of Jupiter at an atmospheric level corresponding to a refractivity of l0- . Two inde- pendent analyses are presented; discrepancies between the results give a measure of the sensitivity to initial assumptions. Analysis of the Io occultation data gives an additional determination of the declina- tion of the center of Jupiter and permits a more precise determination of theJovian oblateness than would have otherwise been possible. For Io, we obtain a radius of 1829+3 km; for Jupiter, the equatorial radius is 71 880+30 km, and the oblateness ( )/R =0.060+0.()0l (mean of the two analyses). The apparent semidiameters of Jupiter at unit distance for the 50% transmission level in the atmosphere are 99100 equatorial and 93"10 polar. The derived oblateness is in good agreement with other determinations of the optical oblateness, and the discrepancy with the dynamical oblateness is confirmed. Possible sources of systematic error are discussed.