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On April 28, 1905, William H. Pickering, who had seven years earlier discovered Phoebe, announced the discovery of a tenth satellite of Saturn, which he promptly named Themis. The photographic plates on which it supposedly appeared, thirteen in all, spanned a period between April 17 and July 8, 1904. However, Pickering was mistaken and Themis does not exist.
Pickering attempted to compute an orbit, which showed a fairly high orbital inclination (39.1° to the ecliptic), fairly large eccentricity (0.23) and a distance (semi-major axis of 1,457,000 km) approximately the same as that of Titan and Hyperion. The period was supposedly 20.85 days, with prograde motion.
Pickering estimated the diameter at 38 miles, but since he also gave 42 miles as the diameter of Phoebe, he was clearly overestimating the albedo; using the modern figure for Phoebe gives Themis a diameter of 200 km.
 Two possible orbits for Themis as calculated by W. H. Pickering Oddly, in April 1861, Hermann Goldschmidt had also believed that he had discovered a new satellite of Saturn between Titan and Hyperion, which he called Chiron. Chiron also does not exist (however, the name was used much later for the comet/asteroid 2060 Chiron).
Pickering was awarded the Lalande Prize of the French Academy of Sciences in 1906 for his "discovery of the ninth and tenth satellites of Saturn".
The true tenth satellite of Saturn (in order of discovery) was Janus, which was discovered in 1966 and confirmed in 1980. Its orbit is far from the supposed orbit of Themis.
There is also an asteroid named 24 Themis.
Themis in fiction John Varley's science fiction novel Titan is set aboard an expedition to Saturn. As they approach the planet and prepare to enter orbit, the astronomer onboard discovers a new moon. At first she believes she has recovered Pickering's lost moon, so she names it Themis.
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