Pliny the Elder refers to "the seven stars, which owing to their motion we call planets, though no stars wander less than they do." Nonnus, the 5th century Greek poet, says in his Dionysiaca, "I have oracles of history on seven tablets, and the tablets bear the names of the seven planets." Planets in the Middle Ages Medieval and Renaissance writers generally accepted the idea of seven planets. The standard medieval introduction to astronomy, Sacrobosco's De Sphaera, includes the Sun and Moon among the planets, the more advanced Theorica planetarum presents the "theory of the seven planets," while the instructions to the Alfonsine Tables show how "to find by means of tables the mean motuses of the sun, moon, and the rest of the planets." In his Confessio Amantis, 14th-century poet John Gower, referring to the planets' connection with the craft of alchemy, writes, "Of the planetes ben begonne/The gold is tilted to the Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon were planets. Even Nicolaus Copernicus, who rejected the geocentric model, was ambivalent concerning whether the Sun and Moon were planets. In his De Revolutionibus, Copernicus clearly separates "the sun, moon, planets and stars"; however, in his Dedication of the work to Pope Paul III, Copernicus refers to, "the motion of the sun and the moon... and of the five other planets." Earth Eventually, when Copernicus's heliocentric model was accepted over the geocentric, Earth was placed among the planets and the Sun and Moon were reclassified, necessitating a conceptual revolution in the understanding of planets. As the historian of science Thomas Kuhn noted in his book, The Structure of Scientific Revolutions: The Copernicans who denied its traditional title 'planet' to the sun ... were changing the meaning of 'planet' so that it would continue to make useful distinctions in a world where all celestial bodies ... were seen differently from the way they had been seen before... Looking at the moon, the convert to Copernicanism ... says, 'I once took the moon to be (or saw the moon as) a planet, but I was mistaken.' Copernicus obliquely refers to Earth as a planet in De Revolutionibus when he says, "Having thus assumed the motions which I ascribe to the Earth later on in the volume, by long and intense study I finally found that if the motions of the other planets are correlated