<p style='text-align: justify;'>Tycho Brahe published this <i>hypotyposis</i> or 'vivid outline' of his world system in chapter 8 of his <i>De mundi aetherei recentioribus phaenomenis</i>, which he produced at his own press on the island of Hven in 1588. The work was originally intended to be the second part of a three-volume series dealing with the nova of 1572, the comet of 1577, and other comets that Tycho had observed; for this reason, it was not immediately distributed commercially, despite being printed in a run of 1500 copies. A number of individuals received copies or partial copies as gifts, and one recipient, Heinrich Rantzau (1526-1598) appears to have further disseminated the diagram alone. The diagram is captioned: '<i>Hypotyposis</i> of the new system of the World recently devised by the author, by which are eliminated both that old Ptolemaic redundancy and inelegance, and also the recent Copernican physical absurdity on the motion of the Earth, and all things correspond most well to the celestial appearances.' A textual account of the world system follows the diagram in the book. The decision to include the world system in this work was not, as is sometimes supposed, because Tycho took the comet of 1577 to have demonstrated the non-existence of the celestial spheres and therefore the possibility of his geoheliocentric world system. Rather, in chapter 8 of <i>De mundi aetherei recentioribus phaenomenis</i>, Tycho assigned the comet a circumsolar path or 'hypothesis', something that required both fluid heavens and a world system in which some bodies orbited the Sun rather than the Earth. He introduced his new world system, therefore, in order to accommodate his account of the motion of the comet. The diagram uses visual conventions that seem familiar to us from the period: lines representing the paths of the planets - indicated by their astrological symbols - around their respective centres of motion, and a narrow band to represent the sphere of fixed stars. Correctly understood, the diagram shows the Earth stationary at the centre of the cosmos and as the centre of motion of the Moon and the Sun, with the Sun as the centre of motion of the five planets. But contemporary reaction indicates that, for individuals used to thinking in terms of spheres and orbs rather than orbits, the diagram was difficult to read. According to Tycho's letter to Rantzau of 13 September 1588 responding to the reported criticisms, Georg Rollenhagen (1542-1609) thought that the diagram allowed for the collision of Mars and the Sun, while Caspar Peucer (1525-1602) was worried by the apparent empty space in the lower half of the figure. Tycho suggested to Rantzau and Peucer that an instrument, displaying the component parts of the world system in motion, would show that both these concerns were illusory: Mars would always remain in orbit around the Sun and the initially empty space would be occupied as the Sun moved around the Earth. Tycho sent such an instrument, or a design for one, to Rantzau, and Peucer acknowledged receipt of a brass example that helped him to better understand Tycho's world system in May 1589. The dissemination and reception of the diagram, and Tycho's response to it, illustrate very well therefore the practical advantages and cognitive limitations of static planetary diagrams vis-Ã -vis mechanical planetaria.</p>
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