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Since the moon is the closest celestial body to earth and the most easily observed with a telescope, it was a natural place for Herschel to begin his search for extraterrestrials. In a letter to a friend, Herschel described how he believed the craters of the moon were Lunarian cities and dwellings (laid out like the Roman “circus” meaning a large ring):
As upon the Earth several Alterations have been, and are daily, made of a size sufficient to be seen by the inhabitants of the Moon, such as building Towns, cutting canals for Navigation, making turnpike roads &c: may we not expect something of a similar Nature on the Moon? – There is a reason to be assigned for circular-Buildings on the Moon, which is that, as the Atmosphere there is much rarer than ours and of consequence not so capable of refracting and (by means of clouds shining therein) reflecting the light of the sun, it is natural enough to suppose that a Circus will remedy this deficiency, For in that shape of Building one half will have the directed light and the other half the reflected light of the Sun. Perhaps, then on the Moon every town is one very large Circus?…Should this be true ought we not to watch the erection of any new small Circus as the Lunarians may the Building of a new Town on the Earth….By reflecting a little on the subject I am almost convinced that those numberless small Circuses we see on the Moon are the works of the Lunarians and may be called their Towns….Now if we could discover any new erection it is evident an exact list of those Towns that are already built will be necessary. But this is no easy undertaking to make out, and will require the observation of many a careful Astronomer and the most capital Instruments that can be had. However this is what I will begin.
Of course this spectacular misapprehension becomes more comprehensible considering how long it took humanity to understand the nature of craters (it wasn’t until the 1960’s that work by astrogeologist Gene Shoemaker, brought about widespread scientific consensus that craters were caused by impacts). Yet Herschel was so devoted to his Lunarians that he came perilously close to inventing findings. As he carefully scrutinized the moon for other living things night after night, imperfect optics and his yearning for alien life sometimes got the best of him. Here is a drawing of a shadow which he perceived might be a forest.
Herschel did not believe that the moon was the only other sphere to support life–he believed that life could be found on all heavenly bodies which are spherical from self-gravitation. And Herschel really meant all such bodies: in the Royal Society’s Philosophical Transactions in 1795 he speculated about beings living on the sun,
The sun…appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or in strictness of speaking, the only primary one of our system….Its similarity to the other globes of the solar system …leads us to suppose that it is most probably inhabited …by beings whose organs are adapted to the peculiar circumstances of that vast globe.
Hershel thought that all of the stars in the universe were like the sun—densely habited and supporting an orbiting network of habited worlds. He wrote “since stars appear to be suns, and suns, according to the common opinion, are bodies that serve to enlighten, warm, and sustain a system of planets, we may have an idea of numberless globes that serve for the habitation of living creatures.” Additionally, Herschel believed that the nebula he observed were other “universes” like our own, each containing innumerable stars—all of which were habited. He was wrong in his interpretation of the particular gaseous nebulae he was looking at, but he was quite right about the existence and nature of other galaxies (although this idea was not proved or accepted until the work of Edwin Hubble).
Poor Herschel’s hunches about extraterrestrial life seem quaint to us now. Couched in boyish exuberance and 18th century idioms, they almost seem risible. Yet Herschel was right about exoplanets and about galaxies beyond our own. He seems to have been the only person of his time to begin to apprehend how vast the universe really is. Thanks to the work of many scientists and explorers we can write off life on the moon and (almost certainly) the sun. However, even with our robot probes and our telescopes, the solar system is shockingly unknown. And beyond the solar system, the large exoplanets we currently know about are strange hot giants we did not expect. The preliminary results of the Kepler mission are beginning to trickle in, and they hint at a profusion of planets (and other things) much more heterogeneous and odd than cosmic uniformitarians might expect. If blogging has taught me one thing, it is not to underestimate Sir Frederick William Herschel (a conclusion I hardly anticipated). So while I chuckle about the perfectly circular cities of the lunarians, I am also keeping an open mind about the immense number of unknown worlds.
Also (as I suspect Sir William felt), I am sad about how many things are simply unknowable.