Themis (hypothetical moon)

Themis was the name of a supposed moon of the planet Saturn. He was never seen again, and is considered non-existent.

On April 28, 1905 William H. Pickering, who discovered Saturn's moon Phoebe seven years earlier, announced the discovery of a tenth Saturn moon known to which he gave the name Themis immediately after the Titan Themis.

In the period from 17 April to July 8, 1904 Pickering took a total of 18 photographic plates, which should show the moon. Pickering interpreted the images incorrectly and believed it to recognize the new moon.

On the basis of his recordings Pickering tried to calculate an orbit for Themis. This showed a rather large inclination ( 39.1 ° to the ecliptic ), as well as a fairly large eccentricity (0.23 ) and a semi-major axis of 1.457 million km. The orbital period should be 20.85 days at a prograde orbit between Titan and Hyperion.

Pickering estimated the diameter to 61 km. As he stated the diameter of Phoebe at 68 km, one can start from a too high value for the albedo. With modern albedo measurements of Phoebe, Themis would have had a diameter of 200 km.

Pickering in 1906 the Lalande Prize of the French Academy of Sciences for the "discovery of the ninth and tenth moon of Saturn " award.

Themis was never observed after Pickering's sighting again, but he was partial to the sixties in astronomical publications mention.

The actual, in the chronology of discoveries, tenth moon of Saturn is Janus, who in 1966 discovered and confirmed in 1980. He is on a much different orbit than that calculated for Themis.

There also exists an asteroid named ( 24) Themis.

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