After a major erruption in 1107, the Helkla volcano in Iceland acquired a reputation throughout Europe as the “Gateway to Hell” and witches were said to gather there at Easter. Kepler explains in his footnotes that he chose Iceland as the setting for the early part of the Somnium because of an apparently ancient myth, recorded by Plutarch, that it was said to be the location of a gateway that allowed travel to other worlds. Fiolxhilde, the mother of Kepler’s hero Duracotus, is described gathering magical herbs on the slopes of the Hekla volcano, and reveals the secret of conversation with interplanetary daemons who can travel to the Moon.

In 1615, Johannes Kepler’s mother Katharina Kepler was arrested and accused of witchcraft, amid rumours that the descriptions in Somnium were a veiled account of the magical arts of Kepler’s mother:

 Katherine Kepler was well known for her vile temper and generally cantankerous disposition, not to mention the fact that the aunt who had cared for her as a child was burned at the stake as a witch. The stage was set, charges were leveled, and in 1615 Katherine Kepler was arrested on suspicion of practicing witchcraft.

A long, tedious, and taxing legal battle resulted: only after five years, part of which his mother spent in prison, was the old woman released; but the damage had been done. Katherine Kepler died in April of 1622 from causes directly attributable to the rigors of her imprisonment; her son had been able to do little significant work while trying to obtain his mother’s release; and the publication of the Somnium, at least for the present, was out of the question.