Information about Themis (moon)

On April 28, 1905, William H. Pickering, who had seven years earlier discovered Phœbe, 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 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 (61 km), but since he also gave 42 miles (68 km) as the diameter of Phoebe, he was clearly overestimating the albedo; using the modern figure for Phoebe gives Themis a diameter of 200 km.



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 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 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.
• 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.

See also

• List of hypothetical astronomical objects

References

• (no references to Goldschmidt's publication found)
• 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

•  • [ e] Moons of Saturn
Generally listed in increasing distance from Saturn. Temporary names in italics.
Ring shepherds	Pan Daphnis Atlas Prometheus S/2004 S 6? S/2004 S 4? S/2004 S 3? Pandora
Co-orbitals	Epimetheus Janus
Inner large (and Trojan)	Mimas Methone Anthe Pallene Enceladus Tethys (Telesto • Calypso) Dione (Helene • Polydeuces)
Outer large	Rhea Titan Hyperion Iapetus
Inuit group	Kiviuq Ijiraq Paaliaq Siarnaq S/2007 S 1
Norse group	Phoebe Skathi S/2007 S 2 Skoll S/2004 S 13 S/2006 S 4 Hyrrokkin Mundilfari S/2006 S 6 S/2006 S 1 S/2004 S 17 Narvi Bergelmir Aegir Suttungr S/2004 S 12 Bestla Farbauti Hati S/2004 S 7 Thrymr S/2007 S 3 S/2006 S 3 Surtur Kari Fenrir Ymir Loge Fornjot
Gallic group	Albiorix Bebhionn Erriapo Tarvos
Rings of Saturn Cassini-Huygens Themis