When Galileo first reported on his careful observations of the Moon's surface using the newly developed telescope, fellow astronomers were astonished to learn that the Moon had geographical features that seemed to match the ones on Earth. Along with mountains, valleys and flat surfaces (which were erroneously assumed to be seas), various other features could be seen that Johannes Kepler, among others, speculated could be the work of the Moon's inhabitants (who were quickly named "Selenites"). Beginning in the 18th century, astronomers such as Johannes Hevelius and Giovanni Riccioli were already debating whether the Moon was inhabited or not with scientific evidence being used to back up both sides of the argument.
As William Herschel pointed out in a 1780 to fellow astronomer Nevil Maskelyne, "who can say that it is not extremely probable, nay beyond doubt, that there must be inhabitants on the Moon of some kind or another?". The exact nature of these lunar inhabitants remained a mystery of course since there was no way to get a closer look without much more powerful telescopes. That didn't prevent "Selenites" from finding their way into the popular consciousness. Even Emmanuel Swedenborg claimed to have encountered them during his spiritual journeys to the various planets in the solar system (he described them as "small, like children of six or seven years old; at the same time they have the strength of men like ourselves"). Cyrano de Bergerac wrote about the Moon's inhabitants in his posthumous work, L'Autre Monde: ou les États et Empires de la Lune (The Other World: or the States and Empires of the Moon). Published in 1657, it was one of the first true science fiction novels.
While de Bergerac's novel and Swedenborg's mystical writings did nothing to resolve the question of whether the Moon was inhabited, more credible scientists eagerly weighed in on the controversy. About the same time, New York journalist R.A. Locke launched his Great Moon Hoax in 1835 with six New York Sun article describing supposedly fantastic discoveries by astronomer John Herschel (probably the best-known astronomer of his time). That included discoveries of unicorns, bipedal giant beavers, winged humanoids, and other fantastic creatures, all made visible by an "immense telescope of an entirely new principle." Though the Moon Hoax was exposed just a few weeks later, the speculation over whether the Moon was inhabited seemed as strong as ever. Edgar Allen Poe came out with a tongue-in-cheek Moon hoax of his own at about the same time.
Though these hoaxes may seem unbelievable now, there was enough scientific speculation about the Moon at the time to make Selenites seem plausible enough. Along with John Herschel, astronomers such as Johann Hieronymus Schroter provided detailed maps of the Moon's surface, complete with certain features which he attributed to cloud formations. Shroter also identified large parts of the lunar surface which he assumed were similar to plains on Earth and even suggested traces of a Selenite city. While the case against lunar life was already strong by the opening of the 19th century, die-hards such as Schoter were still determined to preserve the possibility of Selenites.
Which brings us to the most famous lunar astronomer of his time, at least for a while, Franz von Gruithuisen. Though trained as a medical doctor, von Gruithuisen was one of the most prolific astronomers of the 19th century with hundreds of scientific papers and acting as editor to three astronomy journals at the same time. He was also a keen supporter of Johann Schroter and his Selenites. In 1821, von Gruithuisen wrote a paper defending Shroter's theories about lunar life and even argued for the existence of lunar lakes and a rarified lunar atmosphere. In 1824, he published one of his most famous papers, "Discovery of Many Distinct Traces of Lunar Inhabitants, Especially One of Their Colossal Buildings." Based on various features he had seen on the Moon's surface, including how they changed colour with light, von Gruithuisen argued for a wide belt of lunar vegetation around the equator extending "55 degrees to the south and 645 degrees to the north." He also identified lunar crossing sites along which animals migrated. While von Gruithuisen added a caution that there were limits to what was possible with telescopes, he then described various spots on the Moon's surface which he insisted were roads, walls, fortresses and even cities. His discovery of a star-shaped structure on the Moon led him to speculate that it could be a temple and that the Selenites had their own religion.
The astounding finds von Gruithuisen reported in his paper attracted attention across Europe. The fame from his paper was enough to get him appointed professor of astronomy at the University of Munich though this faded quickly enough. In the meantime, von Gruithuisen wrote other papers arguing for the existence of life on Mercury, Venus and the comets. He was particularly fascinated with Venus and the "ashen light" previous astronomers had seen on the dark part of the planet. Not only did he consider this proof of life, but von Gruithuisen argued that these showed that Venusians held enormous "fire festivals" to mark important occasions in the Venusian civilization.
Unfortunately for von Gruithuisen, his findings about the Moon and Venus were not supported by other astronomers with more powerful telescopes. Carl Friedrich Gauss, the eminent mathematician and astronomer dismissed von Gruithusen's "mad chatter" and Wilhelm Olbers lamented how his Munich colleague could be taken in by "the power of imagination." It seems ironic that Gauss would be so skeptical since one popular legend maintains that he had once proposed sending messages to the Moon's inhabitants by creating giant images in the Siberian plains. Though the story is likely bogus, Gauss was definitely a pluralist as far as life on other planets was concerned. Still, while many of them were open to the possibility of lunar life, the evidence von Gruithuisen was hard for the skeptics to take seriously.
Olbers, in particular, was especially scathing about what von Gruithuisen was claiming to see. In an 1824 letter to a fellow astronomer, he wrote:
"Have you see the representation of Gruithuisen's alleged lunar city and his avenue of trees and his roads in the moon? The power of imagination of the man is large; but nonetheless what he maintains is a city, even though it has no similarity to such, is surely noteworthy if otherwise his drawing, which I have no reason to doubt, is correct"
Wilhelm Olbers had even more reason to complain as his name and Gauss came to be associated with von Gruithuisen in the popular press due to their own ideas about life on other worlds. Considering that von Gruithuisen's claims were getting international press at the time, the success of the Great Moon Hoax in 1835 becomes easier to understand.
But the twilight of the Selenites would arrive soon enough. In an 1834 lecture, Friedrich Bessel denounced the entire notion of Selenites based on his own observations which showed that the Moon had no atmosphere, which meant it had no water either. In discussing why the idea of Selenites was so appealing, Bessel suggested that it was due to a romantic preoccupation in finding aliens like ourselves that kept the idea of lunar life alive. At the same time, more accurate lunar maps became available using far more powerful telescopes than anything von Gruithuisen had. Needless to say, there was no trace of the lunar buildings and roads that von Gruithuisen had reported. If anything, the Moon appeared even more airless and lifeless than his critics had suggested.
As for Franz von Gruithuisen, writing about life on the Moon and Venus largely ruined his scientific credibility despite his incredibly prolific career as an astronomer. By the time of his death in 1852, he was largely remembered only as an extremely gullible observer who allowed his ideas about life on other worlds to overcome his own training as an observer. Today, his chief legacy is the tiny lunar crater named in his honour.
Though science abandoned the notion of Selenites, they were not entirely forgotten courtesy of the rising popularity of science fiction. H.G. Wells featured Selenites his his 1901 novel, The First Men in the Moon, while Edgar Rice Burroughs wrote a series of novels with human-like creatures populating the interior of Earth's moon. As astrophysicists learned more about lunar conditions however, novels featuring Selenites were largely abandoned in favour of more likely sites for extraterrestrial life such as Mars and Europa.
Such is progress...
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