Themis (hypothetical moon)


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, no other astronomer has ever confirmed Pickering's claim.
Pickering attempted to compute an orbit, which showed a fairly high inclination, fairly large eccentricity and a semi-major axis slightly less than that of Hyperion. The period was supposedly 20.85 days, with prograde motion.
Pickering estimated the diameter at, but since he also gave as the diameter of Phoebe, he was clearly overestimating the albedo; using the modern figure for Phoebe gives Themis a diameter of.
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.
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 actual tenth satellite of Saturn 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