Somnium presents a detailed imaginative description of how the Earth might look when viewed from the Moon, and is considered the first serious scientific treatise on lunar astronomy.

Carl Sagan and Isaac Asimov have referred to it as one of the earliest works of science fiction.

The dream begins with Kepler reading a book about Duracotus, an Icelandic boy who is 14 years old.

Duracotus's mother, Fiolxhilde, makes a living selling bags of herbs and cloth with strange markings on them.

After he cuts into one of these bags and ruins her sale, Duracotus is sold by Fiolxhilde to a skipper.

He travels with the skipper for a while until a letter is to be delivered to Tycho Brahe on the island of Hven.

Duracotus is fascinated with astronomy and enjoys the time they spend looking at the night sky.

The summoned daemon tells them, "Fifty thousand miles up in the Aether lies the island of Levania," which is Earth's Moon.

Another concern is the air, so humans have to have damp sponges placed in their nostrils in order to breathe.

The daemons can rush to Volva during a solar eclipse, otherwise they remain hidden in shadows on Levania.

Other details are fictional in nature, such as descriptions of the creatures that inhabit Subvolva and Privolva, plant growth on each side, and the life and death cycle of Levania.

[5] Somnium began as a student dissertation in which Kepler defended the Copernican doctrine of the motion of the Earth, suggesting that an observer on the Moon would find the planet's movements as clearly visible as the Moon's activity is to the Earth's inhabitants.

Nearly 20 years later, Kepler added the dream framework, and after another decade, he drafted a series of explanatory notes reflecting upon his turbulent career and the stages of his intellectual development.

Karl Siegfried Guthke [de] notes that this means that the story predates the invention of the telescope.

Duracotus spends a considerable amount of time working for Tycho Brahe.

During the Privolvan day, some of the water is pumped back to Privolva to protect its inhabitants from the intense heat.