Tidal effects in planetary systems are the main driver in the orbital migration of natural satellites. They result from physical processes occurring deep inside celestial bodies, whose effects are rarely observable from surface imaging. For giant planet systems, the tidal migration rate is determined by poorly understood dissipative processes in the planet, and standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance 1 , implying little migration for outer moons such as Saturn's largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titans orbital expansion rate. We find Titan migrates away from Saturn at 11.3 ± 2.0 cm/year, corresponding to a tidal quality factor of Saturn of Q ' 100, and a migration timescale of roughly 10 Gyr. This rapid orbital expansion suggests Titan formed significantly closer to Saturn and has migrated outward to its current position. Our results for Titan and five other moons agree with the predictions of a resonance locking tidal theory 2 , sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturns moon system. The resonance locking mechanism could operate in other systems such as stellar binaries and exoplanet systems, and it may allow for tidal dissipation to occur at larger orbital separations than previously believed.Saturn is orbited by 62 moons, and the intricate dynamics of this complex system provide clues about its formation and evolution. Of crucial importance are tidal interactions between the moons and the planet. Each moon raises a tidal bulge in the planet, and because Saturn rotates faster than the moons orbit, frictional processes within the planet cause the tidal bulge to lead in front of each moon. Each moon's tidal bulge pulls the moon forward such that it gains angular momentum and migrates outward, similar to the tidal evolution of the Earth-Moon system. However, in giant planets such as Saturn, the dissipative processes that determine the bulge lag 2

A simple but effective approach is proposed for measuring the geometric distortion of a CCD field of view of a ground-based telescope. For three open clusters (M35, M67, and NGC 2324), 425 CCD frames taken by a 1 m telescope at the Yunnan Observatory are used to test this approach. It is found that the geometric distortion pattern depends strongly on the corresponding filter used. The geometric distortion is then used to correct the pixel positions for Phoebe, the ninth satellite of Saturn, and its reference stars imaged in 220 CCD frames taken by the same telescope. The standard deviation of the (O − C; observed minus computed) residuals of Phoebe is significantly improved after correcting the geometric distortions.

Aims. Caviar is a software package designed for the astrometric measurement of natural satellite positions in images taken using the Imaging Science Subsystem (ISS) of the Cassini spacecraft. Aspects of the structure, functionality, and use of the software are described, and examples are provided. The integrity of the software is demonstrated by generating new measurements of the positions of selected major satellites of Saturn, 2013–2016, along with their observed minus computed (O−C) residuals relative to published ephemerides. Methods. Satellite positions were estimated by fitting a model to the imaged limbs of the target satellites. Corrections to the nominal spacecraft pointing were computed using background star positions based on the UCAC5 and Tycho2 star catalogues. UCAC5 is currently used in preference to Gaia-DR1 because of the availability of proper motion information in UCAC5. Results. The Caviar package is available for free download. A total of 256 new astrometric observations of the Saturnian moons Mimas (44), Tethys (58), Dione (55), Rhea (33), Iapetus (63), and Hyperion (3) have been made, in addition to opportunistic detections of Pandora (20), Enceladus (4), Janus (2), and Helene (5), giving an overall total of 287 new detections. Mean observed-minus-computed residuals for the main moons relative to the JPL SAT375 ephemeris were − 0.66 ± 1.30 pixels in the line direction and 0.05 ± 1.47 pixels in the sample direction. Mean residuals relative to the IMCCE NOE-6-2015-MAIN-coorb2 ephemeris were −0.34 ± 0.91 pixels in the line direction and 0.15 ± 1.65 pixels in the sample direction. The reduced astrometric data are provided in the form of satellite positions for each image. The reference star positions are included in order to allow reprocessing at some later date using improved star catalogues, such as later releases of Gaia, without the need to re-estimate the imaged star positions.

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