Themis (hypothetical moon)

On April 28, 1905, the American astronomer William H. Pickering said that he had found a tenth moon of the planet Saturn. Pickering named the moon Themis. No other astronomer has ever seen the moon that Pickering said he saw. Because of this, astronomers do not count Themis as an official moon of Saturn.

Pickering tried to figure out an orbit for Themis. He thought that Themis had an inclination of 39.1° to the ecliptic, with 0.23 eccentricity (0.23) and a semi-major axis distance of 1,457,000 km. This meant that Themis had an orbit like the moons Titan and Hyperion. Pickering believed that it took Themis 20.85 days to orbit Saturn, in a prograde motion.

Pickering thought that the diameter of Themis was about 38 miles (61 km). Pickering had also discovered the moon Phoebe, but modern astronomers know that Pickering made a mistake when he gave 42 miles (68 km) as the diameter of Phoebe. Because astronomers know how Pickering made his mistake, they can say that if Themis existed, it would have a diameter of 200 km.

In April 1861, Hermann Goldschmidt also thought he discovered a new satellite of Saturn between Titan and Hyperion. Goldschmidt called this moon Chiron. The moon Chiron also does not exist, but the name was used much later for the comet/asteroid 2060 Chiron.

In 1906, the French Academy of Sciences gave Pickering the Lalande Prize of the French Academy of Sciences for his "discovery of the ninth and tenth satellites of Saturn".

The actual 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 takes place on 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.
Robert Anton Wilson's novel Schrödinger's Cat trilogy makes frequent reference to Pickering's Moon as a satellite that revolves the "wrong way" around its primary.

References
 Harvard College Observatory Bulletin No. 189, p.1 (April 1905)
 Annals of Harvard College Observatory, vol. 53, no. 9, pp. 173-185 (1905)
 AnHar 53 (1905) 173
 AnHar 61 (1908) 86
 Obs 28 (1905) 12:433
 PASP 18 (1906) 96
 Obs 31 (1908) 8:295
 MNRAS 69 (1909) 215
 Obs 32 (1909) 3:79

Category:Saturn's moons
Category:Hypothetical moons