The Telescope against Copernicus: Star Observations by Riccioli Supporting a Geocentric Universe

I will use the term physical (physical size or physical diameter or physical radius) to refer to the physical extent of a star's globe as calculated by Riccioli or others. This is in contrast to the size or diameter or radius observed by Riccioli or others, either telescopically or via the naked eye.

I borrow this phrase from the book by Harald Siebert of the same title: Die große kosmologische Kontroverse: Rekonstruktionsversuche anhand des Itinerarium exstaticum von Athanasius Kircher SJ (1602–1680) (Stuttgart, 2006).

“Brahé, Tycho”, The penny cyclopædia of the Society for the Diffusion of Useful Knowledge, v (London, 1836), 326.

Blair Ann , “Tycho Brahe's critique of Copernicus and the Copernican system”, Journal for the history of ideas, li (1990), 355–77, p. 364.

Drake Stillman , Discoveries and opinions of Galileo (Garden City, NY, 1957), 47, note 16.

Schofield Christine , “The Tychonic and semi-Tychonic world systems” in The general history of astronomy: Planetary astronomy from the Renaissance to the rise of astrophysics, Part A , ed. by Taton René Wilson Curtis (Cambridge, 1989), 41.

Drake , op. cit. (ref. 5), 47.

Kepler Johannes , transl. by Wallis C. G. , Epitome of Copernican astronomy & Harmonies of the world (Amherst, NY, 1995), 46.

Van Helden Albert , Measuring the universe: Cosmic dimensions from Aristarchus to Halley (Chicago, 1985), 89.

Grant Edward , Stars, planets, and orbs: The medieval cosmos, 1200–1687 (Cambridge, 1996), 448. “The fixed stars appear as dimensionless points” are Van Helden's words. Kepler (ref. 8) says “Skilled observers deny that any magnitude as it were of a round body can be uncovered by looking through a telescope: Or rather, if a more perfect instrument is used, the fixed stars can be represented as mere points, from which shining rays, like hairs, go forth and are spread out”. Kepler believed this supported his calculations and experiments which showed that the stars were far smaller and dimmer than the Sun (according to Kepler, the Sun viewed from a fixed star would appear brighter than any fixed star seen from Earth). Kepler's tiny stars would help answer Tycho's objection to Copernicus. See Van Helden , op. cit. (ref. 9), 88–89.

Grant Edward , “In defense of the Earth's centrality and immobility: Scholastic reaction to Copernicanism in the seventeenth century”, Transactions of the American Philosophical Society, n.s., lxxiv (1984), 1–69.

Feingold Mordechai (ed.), Jesuit science and the Republic of Letters (Cambridge, MA, 2003).

Drake , op. cit. (ref. 5), 100.

Drake , op. cit. (ref. 5), 137.

Ondra Leos , “A new view of Mizar”, Sky and telescope, July 2004, 72–5.

Galilei Galileo , transl. by Finocchiaro M. A. , “Galileo's reply to Ingoli: 1624”, in The Galileo affair, by Finocchiaro M. A. (Berkeley, CA, 1989), 154–97, pp. 167, 180.

Galilei Galileo , transl. by Drake Stillman , Dialogue concerning the two chief world systems — Ptolemaic & Copernican, 2nd edn (Berkeley, CA, 1967), 359–60.

A full discussion of telescopic star disk observations over two centuries can be found in Graney C. M. Grayson T. P. , “On the telescopic disks of stars: A review and analysis of stellar observations from the early 17th through the middle 19th centuries”, Annals of science (in press).

Airy G. B. , “On the diffraction of an object-glass with circular aperture”, Transactions of the Cambridge Philosophical Society, v (1835), 283–91, p. 288.

Note that according to Airy's discussion, neither of the stars mentioned would have rings bright enough to see, as the peak of the brightest ring in an Airy pattern is less than 1/10 the intensity of the “central light”.

Galileo , op. cit. (ref. 17), 359–60.

Graney C. M. , “But still, it moves: Tides, stellar parallax, and Galileo's commitment to the Copernican theory”, Physics in perspective, x (2008), 258–68.

Graney C. M. , “Seeds of a Tychonic revolution: Telescopic observations of the stars by Galileo Galilei and Simon Marius”, Physics in perspective, xii (2010), 4–24.

Riccioli G. B. , Almagestum novum (Bologna, 1651). A high-resolution on-line version is available at <http://www.e-rara.ch/zut/content/pageview/140190>.

In reading this, a sort of “blink comparison”, in which one eye observes the paper while the other observes the image in the telescope, or a similar method, comes to mind.

Note that (34.5/44) × 200 = 157. Riccioli also refers to Saturn's diameter as 35″; (35/44) × 200 = 159. Riccioli's numbers are approximate, and, as the reader will see in Tables 1–4, full of rounding, typographical, and other editing errors.

The difference between Riccioli's 18″ diameter for Sirius and the 10″ diameter he ascribes to Hortensius is not sufficient to make a difference in the argument, as Riccioli will implicitly note shortly in his criticism of Galileo. Riccioli probably overestimated the telescopic size of Jupiter; according to the computer program Stellarium, Jupiter's apparent diameter at the time was 34″, considerably smaller than Riccioli's value of 44″. Since Riccioli measured stellar diameters in terms of Jupiter, this would inflate his star-measurements. Thus the difference between Riccioli's measurements and those of Hortensius is probably not that great.

van Lansberge Philip , Uranometriae libri tres (Middleburg, 1631), 130–4.

Riccioli actually says “diameter” rather than “radius”. However, Riccioli's tables contain numerous errors where diameters are stated but radii given, and the telescopic star sizes he gives are more consistent with diffraction pattern calculations for an aperture of one-quarter inch radius. It seems probable that the same diameter/radius error exists here.

The reader may question whether Riccioli is merely choosing an arbitrary aperture size: Why stop the restriction of aperture once the rings are scraped off? If restricting the aperture to ¼-inch radius renders a truer star image than less restriction, would not still further restriction render a still truer image? The reader should keep in mind the appearance of a star image when a refracting telescope is being focused. When a star is optimally focused, its image is as small and intensely bright as possible. Further adjustment in either direction (either lengthening or shortening the distance from object lens to eye lens) will result in the image's swelling into a larger and less intense disk of light. Now consider aperture restriction. Owing to the phenomenon of diffraction, restriction of a telescope's aperture enlarges the spurious disk, but reduces its intensity — See Airy, op. cit. (ref. 19). In general, aperture restriction beyond that needed to eliminate diffraction rings results in the star image's becoming a larger and less intense disk of light. Thus an astronomer such as Riccioli probably viewed aperture restriction as a sort of secondary “focusing” for stars — One restricts the aperture until no rings are seen, but no further so as not to enlarge and apparently defocus the disk. Because there is a fixed ratio between the intensities of the rings and the peak of the central disk, various observers should reach fairly consistent results when observing a bright star such as Sirius, differing mostly in when they judge the last ring to have been fully “scraped off”. However, the interplay between aperture, intensity, disk and ring sizes, and the limits of sensitivity of the eye (which has two different types of light detecting cells of greatly differing sensitivities) is complex. Faint stars such as Alcor will show no rings at all in a small telescope — How did Riccioli treat them? A full treatment of the subject of what affects disk size, in light of Airy's paper, is beyond the scope of this paper but is largely available in Graney C. M. , “17^th century photometric data in the form of telescopic measurements of the apparent diameters of stars by Johannes Hevelius”, Baltic astronomy , xvii (2009), 253–63; and in Graney Grayson , op. cit. (ref. 18). The purpose of these remarks is only to illustrate that Riccioli's statement that there is an approximate optimum aperture for viewing the disk of a star, scraped of its adventitious rays, is not inconsistent with what is understood concerning star images seen through a small aperture telescope.

Hortensius M. , Dissertatio de Mercurio in Sole viso (Leiden, 1633), 60–4. A high-resolution on-line version is available via Google Books.

Whatton A. B. Rev. , Memoir of the life and labours of the Rev. Jeremiah Horrox, to which is appended a translation of his celebrated discourse upon the transit of Venus across the Sun (London, 1859), 198–9.

Halley E. , “Some remarks on a late essay of Mr Cassini, wherein he proposes to find, by observation, the parallax and magnitude of Sirius”, Philosophical transactions, xxxi (1720), 1–4.

Huygens C. , Systema Saturnium (The Hague, 1659), 7.

Hevelius J. , Mercurius in Sole visus (Gdansk, 1662), 92–5. The telescopic star diameters measured by Hevelius are more in line with those of Galileo or Hortensius than Riccioli (Hevelius measures Sirius to be just over 6″). The Hevelius measurements also appear to be quite precise, agreeing very closely to Airy's theory. See Graney , op. cit. (ref. 30). Note that Hevelius includes this information despite his publication of the occultation observations of Horrocks in the same volume.

von Guericke Otto , Experimenta nova (ut vocantur) Magdeburgica de vacuo spatio / The new (so-called) Magdeburg experiments of Otto von Guericke, transl. by Glover Margaret Ames Foley (Dordrecht, 1994).

Von Guericke generally argues for stars' having widely ranging sizes and distances. To make his arguments he attempts to discredit measurements of star sizes, including those made telescopically by Riccioli and Hortensius, both of whose measurements he lists in a table in Book I, Chapter 30 of Magdeburg experiments. However, he makes his arguments against the validity of measured star sizes simply on the basis of naked eye observations of candles and torches. The telescopic measurements of Riccioli, Hortensius, and Galileo, all of which are in the range of seconds of arc, are included with much larger naked-eye measurements given in minutes of arc. Von Guericke settles on a size for Sirius by simply adding a second of arc to a size provided by Galileo. It is not clear that he has performed any telescopic measurements of stars himself. See Book I, chaps. 26–30 and Book VII, chaps. 1–2. So it is worth noting that even though von Guericke gives himself a great deal of latitude in setting star sizes and distances, and says that “it should be understood that nothing certain can be said about the size of stars” (Book VII, Chapter 2, 357), he still admits that those stars we can see can be quite large “as Riccioli declared” — As much as “five, ten, even one hundred etc. times greater” than the Sun (Book VII, Chapter 2, 359). Von Guericke cannot fully dismiss Riccioli and the question of large physical sizes of stars, which he might have been able to do on the basis of work such as that of Horrocks.

In fact, even at the time of Riccioli a thorough investigation of parallax would suggest a value less than 10″. See Graney , opera cit . (refs 22 and 23).

A decade and a half after the Almagestum novum, in his Astronomia reformata, Riccioli devotes less emphasis to the issue of the physical sizes of stars. He again publishes his table of telescopically measured star diameters (essentially the same as Table 1), but without the extensive tables of and commentary on the physical sizes of stars as they relate to the Copernican system (although he does mention the issue). He includes a table of telescopic star diameters measured by other astronomers, but it includes neither Hevelius's measurements from Mercurius in Sole visus (ref. 35) nor Horrocks's observations from the same. He quotes from Systema Saturnium (ref. 34) regarding measuring the diameter of planets; perhaps after reading Huygens he was less willing to invest time in the fixed stars issue. At any rate, while it is conceivable that Riccioli's anti-Copernican argument could have been strengthened by advancing science before it was weakened by it, the Astronomia reformata gives no indication that this was the case. See Astronomia reformata (Bologna, 1665), Book 10, Chapter 1 (pp. 353–4) and Book 10, Chapter 8 (pp. 359–60).

Graney , op. cit. (ref. 23), 19–22.

Bruce Eastwood, review of Grant's Edward “In defense of the Earth's centrality and immobility: Scholastic reaction to Copernicanism in the seventeenth century”, Isis, lxxvi (1985), 378–9.

A final note on this topic: Riccioli's observations of stars have not completely escaped notice. Juan Casanovas (“The problem of the annual parallax in Galileo's time” in The Galileo affair: A meeting of faith and science (Proceedings of the Cracow Conference, 24–27 May 1984), ed. by Coyne G. V. Heller M. Zyncincski J. (Vatican City, 1985), 67–74) mentions that Riccioli measured the telescopic diameter of Sirius and made stellar distance and physical size calculations based on parallax (p. 68). Casanovas even discusses how Galileo believed “he could see the real disk of a star” (p. 72). However, Casanovas cites Kepler (again the same quote as previously mentioned), and concludes that “it was realized that the angular diameters of stars were not perceptible with telescopes, which were seen to be still very imperfect” (p. 73). Early telescopes could be far from imperfect — See Greco V. Molesini G. Quercioli F. , “Optical tests of Galileo's lenses”, Nature , ccclviii (1992), 101. As mentioned in ref. 35, Hevelius produced very precise stellar observations with his telescope, and Galileo did the same; see Graney C. M. , “On the accuracy of Galileo's observations”, Baltic astronomy , xvi (2007), 443–9. In The assayer Galileo defends the validity of telescopic views of stars, attributing any possible minor differences between a telescope's view of the Moon and its view of the stars to merely changes in the required focus — See Drake , op. cit. (ref. 5).