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Themis (hypothetical moon)

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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".


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.