Themis (hypothetical moon)

Lore & Background

William H. Pickering announced the discovery of Themis on April 28, 1905, naming it after the Greek goddess of divine law and order. The thirteen photographic plates on which it supposedly appeared spanned from April 17 to July 8, 1904. Pickering attempted to compute an orbit, which showed a fairly high inclination of 39.1° to the ecliptic, a fairly large eccentricity of 0.23, and a semi-major axis of 1,457,000 kilometers (905,000 miles), slightly less than that of Hyperion. The period was supposedly 20.85 days, with prograde motion.

Pickering estimated Themis's diameter at 38 miles (61 km), based on his assumptions that Themis's albedo was the same as Titan's, and that Titan's diameter was about 2,300 miles (3,701.4912 km). Today, Titan's diameter is known to be closer to 3,200 miles (5,149.9 km). The same assumptions led him to a diameter of 42 miles (68 km) for Phoebe, far less than Phoebe's true diameter of roughly 132 miles (213 km).

No other astronomer ever confirmed Pickering's claim. In April 1861, Hermann Goldschmidt had also believed he had discovered a new satellite of Saturn between Titan and Hyperion, which he called Chiron; it does not exist, though the name was used much later for the large comet and centaur 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, discovered in 1966 and confirmed in 1980.

Reader's Guide

Themis is significant as a notable hypothetical astronomical object from the early 20th century, illustrating the challenges of astronomical observation before modern techniques. William H. Pickering's claim, though unconfirmed, was taken seriously enough to earn him the Lalande Prize in 1906. Themis's non-existence highlights the difficulty of verifying faint objects near bright planets, and its story parallels the earlier unconfirmed satellite Chiron announced by Hermann Goldschmidt in 1861. Themis has also appeared in science fiction, including Philip Latham's 1953 novel Missing Men of Saturn, John Varley's 1979 novel Titan, Robert Anton Wilson's Schrödinger's Cat trilogy, Nelson S. Bond's 1943 story 'The Ordeal of Lancelot Biggs,' and the series Gemini Home Entertainment. These fictional uses reflect the enduring fascination with lost or hidden moons. Themis's legacy is as a cautionary tale in the history of astronomy and a recurring motif in speculative fiction.

Did You Know?

• William H. Pickering announced Themis on April 28, 1905, based on thirteen photographic plates taken between April 17 and July 8, 1904.
• No other astronomer ever confirmed Pickering's claim of Themis.
• Pickering estimated Themis's diameter at 38 miles (61 km) using assumptions about Titan's albedo and diameter that later proved incorrect.
• The actual tenth satellite of Saturn (in order of discovery) was Janus, discovered in 1966 and confirmed in 1980.