Themis (hypothetical moon) facts for kids
On April 28, 1905, an American astronomer named William Henry Pickering announced an exciting discovery. He believed he had found a tenth moon orbiting the planet Saturn. Pickering named this new moon Themis.
However, no other astronomer has ever been able to see the moon that Pickering described. Because of this, Themis is not considered an official moon of Saturn today. It remains a hypothetical, or "imagined," moon.
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What Pickering Thought About Themis's Orbit
Pickering tried to figure out how Themis would move around Saturn. He thought it would have an inclination (how tilted its path is) of 39.1 degrees compared to the ecliptic (the plane where Earth orbits the Sun). He also believed it had an eccentricity (how stretched out its orbit is) of 0.23.
The distance of its semi-major axis (half of the longest diameter of its orbit) was estimated at 1,457,000 kilometers. This meant Themis's path would be similar to Saturn's moons Titan and Hyperion. Pickering calculated that Themis would take 20.85 days to complete one trip around Saturn, moving in a prograde (forward) direction.
How Big Was Themis Supposed to Be?
Pickering thought Themis was about 38 miles (61 kilometers) wide. He had also discovered another moon of Saturn called Phoebe. Modern astronomers know that Pickering made a mistake when he measured Phoebe's size, saying it was 42 miles (68 kilometers) wide.
Because scientists understand Pickering's mistake, they can guess that if Themis really existed, it would actually be much larger, around 200 kilometers (124 miles) across.
Other Imagined Moons of Saturn
In April 1861, another astronomer named Hermann Goldschmidt also thought he found a new moon of Saturn. He believed it was located between Titan and Hyperion and called it Chiron. This moon also turned out not to exist. However, the name Chiron was later used for a different object: a comet or asteroid known as 2060 Chiron.
Awards and Real Discoveries
In 1906, the French Academy of Sciences gave Pickering the Lalande Prize. This award was given for his "discovery of the ninth and tenth satellites of Saturn," even though the tenth one (Themis) was never confirmed.
The actual tenth moon of Saturn to be discovered was Janus. It was first spotted in 1966 and officially confirmed in 1980. Janus's orbit is very different from where Themis was thought to be.
There is also an asteroid named 24 Themis, but it is not related to Saturn's moons.
Themis in Stories
- In John Varley's science fiction novel Titan, a space mission travels to Saturn. An astronomer on board discovers a new moon. At first, she thinks it's Pickering's lost moon, so she names it Themis.
- Robert Anton Wilson's novel Schrödinger's Cat trilogy often mentions Pickering's Moon as a satellite that orbits "the wrong way" around its planet.
Images for kids
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Artist's impression of Themis
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Two possible orbits for Themis as calculated by W. H. Pickering
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See also
In Spanish: Temis (luna hipotética) para niños
