Orrery on display in Kassel at the Astronomisch-Physikalisches Kabinett and in Dresden at the Mathematisch-Physikalischer Salon. In "De revolutionibus orbium coelestium", published in Nuremberg in 1543, Nicolaus Copernicus challenged the Western teaching of a geocentric universe in which the Sun revolved daily around the Earth. He observed that some Greek philosophers had proposed a heliocentric universe. This simplified the apparent epicyclic motions of the planets, making it feasible to represent the planets' paths as simple circles. This could be modelled by the use of gears. Tycho Brahe's improved instruments made precise observations of the skies (1576–1601), and from these Johannes Kepler
The Book of the Short Sun least "Locus: The Magazine of the Science Fiction & Fantasy Field" considered the three "Short Sun" volumes separately for annual "Best Novel" awards. The "Short Sun" of the title is an ordinary star, in contrast to the "Long Sun" of the Whorl where the narrator grew up. Most of the story takes place in a star system with two habitable planets, Blue and Green, which lend their names to the first two volumes. The Whorl of volume three's title is the generation ship setting of "Long Sun". The Tor hardcover editions (see table) were almost 1200 pages long in sum.