How did a Copernican deal with 'star size' objection to Copernican theory articulated by Tycho Brahe (1546-1601)? According to Giovanni Battista Riccioli (1598-1671), a Copernican did so by appealing to divine power: an infinite and omnipotent God can make stars as large as He desires; such stars are nothing compared to God. A fascinating illustration of this approach to Copernican theory's 'star size' problem can be found in work of Dutch Copernican Philips Lansbergen (1561-1632).Tycho's 'star size' objection was straightforward. Fixed stars appear to naked eye to have measurable sizes. Tycho measured first magnitude stars to be about two arc minutes in apparent diameter, and lesser stars to be progressively smaller - values that generally agreed with earlier measurements.1 By geometry, more distant an object of a given apparent size is, physically larger it must be. Therefore, great Copernican distance to fixed stars, required in that theory to explain lack of observable annual stellar parallax, translated into large physical sizes. Tycho's measurements and calculations indicated even a third magnitude Copernican star to be as physically large as orbit of Earth - dwarfing even Sun. By contrast, in Tycho's geo-heliocentric theory, stars lay just beyond Saturn (Figure 1), and had reasonable physical sizes (consistent with other celestial bodies - Figure 2).Here was a weighty anti-Copernican argument. As Albert van Helden has put it: 'Tycho's logic was impeccable; his measurements above reproach. A Copernican simply had to accept results of this argument.2 The telescope is often said to have dissolved this argument, because telescope made necessary a re-estimation of true diameter of fixed stars, which were now known to be smaller than they appear to naked eye.3 However, early telescopes produced images of stars that had appearance of distinct disks, whose spurious nature was not understood at time.4 The telescope did reveal stars to have smaller diameters than those Tycho measured non-telescopically, but it also increased Copernican theory's sensitivity to parallax. The net result was that Tycho's objection remained. Simon Marius, Riccioli, and even Copernican Martinus Hortensius all noted problem that telescopically measured star diameters posed for Copernican theory; even Galileo's telescopic star observations challenged Copernican theory, although Galileo did not address this.5How could a Copernican respond to Tycho's objection? According to Riccioli, by appeal to power of God.6 Indeed, English Copernican Thomas Digges wrote in 1576 of starry heaven with its stars far excelling our sun as being the palace of felicity and habitacle of elect. The German Copernican Christoph Rothmann defended giant stars to Tycho in 1590 by invoking imagery of a palace of God and claiming that size was irrelevant to an infinite creator.7 Responding to Rothmann, Tycho had rejected such reasoning:On what is such an assertion based? Where in nature do we see Will of God acting in an irregular or disorderly manner? In nature where all things are wellordered in all ways of time, measure, and weight? In nature where there is nothing empty, nothing irrational, nothing disproportionate or inharmonious. Consider 700,000 or more Earth semidiameters being between Saturn and fixed stars in Copernican hypothesis. Consider those same fixed stars each being as large as whole Orbit of Earth (and some larger still) and thus dwarfing Sun, luminary and centre of motion for all planets. These are same fixed stars which are noted as least of heavenly lights in account of Creation of World. This is empty, irrational, disproportionate and inharmonious. Is such a disproportionate universe reasonable?8Tycho went on to say that by Divine providence such an ageometrical, asymmetrical, disorderly, and most unworthy method of philosophizing will go away. …
Read full abstract
Dec 1, 2022
William Morris 
Open Access
