Johannes Kepler «Somnium» circa 1600.

The Somnium, circa 1600.

One of the most important books in the history of science, Kepler's long-overlooked Somnium made a significant contribution to the study of astronomy. Its unique twofold nature, combining a serious scientific treatise on lunar astronomy with a fictional narrative about a trip to the moon puzzled seventeenth-century readers as well as succeeding generations, and the work lapsed into obscurity.

The Somnium begins like a classical legend and relates the author’s 'dream' about the adventures of a young man, Duracotus, a native of an island called Thule by the ancients, Iceland by seventeenth-century Europeans. Duracotus’ father, a fisherman by trade, died at the extremely advanced age of 150, but the child was still too young to have any recollection of him. Fiolxhilde, the mother, is a 'wise woman,' who supports both her son and herself by gathering herbs which are then cooked, stuffed in little bags of goatskin, and sold at a nearby port to sailors. The bags supposedly harbor mysterious lucky charms and the healing powers required by seamen on the long and always dangerous voyages across the north Atlantic. One day, out of curiosity Duracotus cut open one of the bags his mother intended to sell to a ship’s captain, scattering its contents on the ground. In a fit of anger Fiolxhilde’s temper got the best of her and she sold her son to the captain in place of the lost herbs.


The following day, the captain set sail for Norway but he stopped in Denmark to deliver a letter from a bishop in Iceland to the astronomer Tycho Brahe, who then resided on the island of Hveen in the Sund between Copenhagen and Elsinore Castle. Duracotus became quite ill during the voyage, apparently he carried no bag of his mother’s charms, and he was put ashore when Tycho’s letter was delivered. The astronomer questioned the boy at some length, considered him to be quite intelligent, and undertook to train him in the science of astronomy. Duracotus’ response is enthusiastic: "I was delighted beyond measure by the astronomical activities, for Brahe and his students watched the moon and the stars all night with marvelous instruments.
After spending five years in Tycho’s company Duracotus took his master’s leave and sailed for home. He found Fiolxhilde much as she was when he left, except that the old woman had suffered terribly as a result of her impetuosity and was overjoyed to see her son alive and well. A number of long discussions ensued during which Fiolxhilde expressed happiness over Duracotus’ acquaintance with the new science of the stars. She confesses to her own special knowledge of the heavens and the fact that her teacher is none other than the "Daemon of Lavania"—the spirit of the moon. "Most of the things which you saw with your own eyes or learned by hearsay or absorbed from books, he related to me as you did." The mother then reveals her ultimate secret: it is possible, with the assistance of the Daemon, to travel to Lavania and, quite predictably, she asks her son to accompany her on just such a lunar voyage. Duracotus consents and "as soon as the sun set below the horizon, and was in conjunction with the planet Saturn in the sign of the Bull, Fiolxhilde summoned the Daemon and seated herself next to her son who covered their heads with a blanket. Within a few moments the journey of 'fifty thousand German miles' had begun, up through the ethereal regions to the moon.


A second, and more important source of inspiration for Kepler’s moon voyage was Plutarch’s The Face on the Moon, which Kepler read in 1595. It is a symposium of Greek scientific thought that includes the views of Hipparchus, Aristotle, and Aristarchus of Samos. Extensive speculation on the lunar environment as a possible home for life is presented; and Plutarch even relates the story of a mythical traveler—a Greek Duracotus—who sails to an island whose residents have knowledge of the passage to the moon.12 Kepler now had the classical precedent he lacked during his student days: he even hoped to publish translations both of Lucian’s and Plutarch’s work with the Somnium to show his debt to these classical writers, and hopefully blunt potential criticism of his own moon voyage.13 It was a task he did not complete.


While Kepler’s method of flight to the moon is not markedly different from that outlined by Lucian, and although much of his inspiration for lunar exploration is undeniably Plutarchian, the Somnium represents a sharp break with classical tradition; the first intimation of which occurs during the voyage itself. We are informed that the flight of four hours is most difficult and fraught with the greatest danger to life. Only those who are slender of body are acceptable, thus ruling out most German males whose general corpulence was apparently distasteful to the slender Kepler. In jest Kepler carried the matter further by pointing out the Daemon’s preference for «dried-up old women, experienced from an early age in riding he-goats at night or forked sticks or threadbare cloaks.» It was to prove a most costly joke for, as we shall see, it later backfired on its author whose own mother was accused of practicing witchcraft by superstitious neighbors and nearly burned at the stake by the authorities.

The take-off for the moon hits the traveler as a severe shock, «for he is hurled just as though he had been shot aloft by gunpowder to sail over mountains and seas.» In order to counteract what Isaac Newton would later define as the force of gravity, the moon voyagers are put to sleep with the aid of opiates and their limbs are arranged in such a way that their bodies will not be torn apart by the force of acceleration.14 Since breathing is inhibited by the swift passage of extremely cold air through the nostrils, damp sponges are applied to the face. Within a short time the speed of flight becomes so great that the body involuntarily rolls itself up into a ball like an endangered spider and we are carried along almost entirely by our will alone, so that finally the bodily mass proceeds toward its destination of its own accord. Kepler had introduced the concept of inertia to the physical sciences and had extended its operation into the heavens.


Kepler anticipates another major obstacle to the moon voyager when he observes that we agreed not to begin «until the moon begins to be eclipsed on its eastern side. Should it regain its full light while we are still in transit, our departure becomes futile.» In other words, Kepler knew that once outside the protective blanket provided by the earth’s atmosphere, humans could not survive the resulting solar bombardment: the flight must begin at the critical moment when the sun is behind the earth or at a point directly opposite the point of take-off. During a lunar eclipse the earth’s shadow would provide the tunnel of darkness required to protect the vulnerable moon voyager; and it is not by accident that the maximum duration of such an eclipse is four and one-half hours, just one-half hour more than the duration of the voyage itself.15 A further indication of Kepler’s mastery of Copernican astronomy is his understanding that since the earth and the moon are both in motion, the shortest route to the latter would not be the straight line advocated by such ancient writers of mythology as Lucian, but a trajectory from earth to a point in space where the moon and the lunar voyagers would arrive simultaneously.16

«Campanella wrote a City of the Sun. What about my writing a 'City of the Moon?' Would it not be excellent to describe the cyclopic mores of our time in vivid colors, but in doing so—to be on the safe side—to leave this Earth and go to the Moon?» ~ Johannes Kepler.

Kepler's model to explain the relative distances of the planets from the Sun in the Copernican System.

Following an account of the painful family circumstances and risks attending the posthumous publication of Somnium in 1634, this essay contends that the work marks the beginning of a new era. After an initial tribute to the classicists, the modern scientist takes over. The Daemon of Lavania is nothing less than Kepler’s own subtly masked voice, speaking with authority about the unlimited possibilities of science. Gone is the fantasy-utopian world of Lucian and Campanella; in its place is an imaginative modern work anchored in fact and rich in rational scientific theory.

And if Kepler’s small-scaled fictional work was overlooked by historians of science for over 350 years, writers of cosmic voyages during the seventeenth, eighteenth, and nineteenth centuries did not make the same mistake. The Somnium was known to Jules Verne, H. G. Wells, and, I believe, to such contemporary writers as Arthur C. Clarke. Kepler opened the way for a new vision of the universe as a home to a plurality of worlds; indeed, Kepler’s Dream may be seen as the 'fons et origo' of modern science fiction.