It is difficult to measure the length of Saturn's day since its visible surface rotates at different speeds at different latitudes, being fastest in the equatorial zone. During their fly-bys the Voyagers detected a periodicity in the rhythm of the radio emission, and this was taken to be the rate at which the planet's core was rotating. However, in 1997 the Paris Observatory announced a different radio periodicity. M.D. Desch of the Goddard Space Flight Center analysed the data collected by Cassini's Radio and Plasma Wave Spectrometer between 29 April 2003 and 10 June 2004 and found a periodicity of 10 hours 45 minutes 45 seconds ( + 36 seconds); some 1 per cent longer than the Voyager time.283 As team leader Donald Gurnett of the University of Iowa put it, ''I don't think any of us could conceive of any process that would cause the
rotation of the entire planet to actually slow down.'' Alex Dessler of the University of Arizona at Tucson had argued that the magnetic fields of the gas giants more resemble that of the Sun than Earth.284,285 The Sun's magnetic field does not rotate as a rigid body; its rotation period varies with latitude, being fastest at the equator. He suggested that the change in the periodicity meant that the part of Saturn's magnetic field that controlled the radio emissions had migrated to a higher latitude. If the magnetic axis had been significantly inclined to the rotational axis, it would have been simple to measure the rate at which the magnetic axis precessed. Nevertheless, by the half-way point in the tour the Dual-Technique Magnetometer had noted a periodicity in the magnetic field itself of 10 hours 47 minutes 6 seconds ( + 40 seconds); even slower than the current radio periodicity.286
On 24 July 2006, Cassini imaged the trailing hemisphere of Dione from a range of 263,000 kilometres. This recreated at higher resolution a Voyager-era perspective that showed the mysterious elliptical feature named Amata amid a knot of wispy streaks to be illusory. Revolution 27 began with apoapsis at 49.0 planetary radii on 4 August, and solar conjunction followed between 5 and 9 August. The plume from Enceladus's south pole was imaged by forward-scattered sunlight from a range of 2.2 million kilometres on 11 August. There was a non-targeted fly-by of Titan on 18 August at 340,595 kilometres. Revolution 28 began at 48.8 planetary radii on 28 August. The steepening of Cassini's orbital plane continued on 7 September with the T17 fly-by at 1,000 kilometres, during which the radar imagery revealed an impact crater of about 30 kilometres diameter with a central peak.
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