On April 28, 1905, William H. Pickering , who had discovered the Saturn moon Phoebe seven years earlier , announced the discovery of a tenth Saturn moon, which he immediately named Themis after the titan Themis .
In the period from April 17 to July 8, 1904, Pickering took a total of 18 photographic plates that were supposed to show the moon. Pickering misinterpreted the images and believed he recognized the new moon. Using his recordings, Pickering tried to calculate an orbit for Themis. This showed a fairly large inclination of 39.1 ° to the ecliptic , also a fairly large eccentricity of 0.23 and a large semi-axis of 1,457,000 km. The orbital period should be 20.85 days in a progressive orbit between Titan and Hyperion . Pickering estimated the diameter to be 61 km. Since he gave the diameter of Phoebe at 68 km, one can assume that the value for the albedo is too high . With modern albedo measurements from Phoebe, Themis would have been 200 km in diameter.
Themis was never observed again after Pickering's sighting, but he was sometimes mentioned in astronomical publications until the 1960s.
The actual tenth moon of Saturn in the chronology of discoveries is Janus , which was discovered in 1966 and confirmed in 1980. It is in a significantly different orbit than that calculated for Themis.
There is also an asteroid named (24) Themis .
- Harvard College Observatory Bulletin No. 189, p.1 (April 1905), bibcode : 1905BHarO.189 .... 1P
- Annals of Harvard College Observatory, vol. 53, no. 9, pp. 173-185 (1905), bibcode : 1905AnHar..53..173P
- AnHar 53 (1905) 173
- AnHar 61 (1908) 86