On the telescopic disks of stars - a review and analysis of stellar observations from the early 17th through the middle 19th centuries

Christopher M. Graney Jefferson Community & Technical College, 1000 Community College Drive, Louisville, KY 40272 (USA) 502-213-7292 christopher.graney@kctcs.edu

Timothy P. Grayson Independent Scholar timotpg@gmail.com

Since the dawn of telescopic astronomy astronomers have observed and measured the “spurious” telescopic disks of stars, generally reporting that brighter stars have larger disks than fainter stars.
Early observers such as Galileo Galilei interpreted these disks as being the physical bodies of stars; later observers such as William Herschel understood them to be spurious; some, such as Christian Huygens, argued that stars show no disks at all. In the early 19th century George B. Airy produced a theoretical explanation of star images sufficient to explain all historical observations, but astronomers were slow to fully recognize this. Even today conventional wisdom concerning stars and telescopes stands at odds to both historical observations and Airy‟s theory.
We give a detailed analysis of both historical observations and Airy‟s theory, illustrating how Airy‟s theory explains the historical observations, from Galileo to Huygens to Herschel. We argue that the observations themselves appear in all cases to be valid and worth further study. 2

3 Introduction The “spurious” telescopic disks of stars have been observed and measured since the very beginning of telescopic astronomy. And since that beginning a consistent theme has been present in the work of those astronomers who possessed both the observing skills and the quality instruments needed to see those disks – brighter stars have larger disks than fainter stars. Early observers such as Galileo (Finocchiaro 1989: 167-168, 173-174, 180) interpreted these disks as being the physical bodies of stars. Later observers like Halley (1720: 3) and Herschel (1805: 40-44) understood the telescopic disks of stars to be semi-spurious – they were partly a product of the telescope, but partly a product of the star itself. When George Airy produced a theoretical explanation for these disks (1835: 283-291), astronomers in the mid-19th century were slow to reconcile this theory, which said that all stellar images were characterized only by the aperture of the telescope in question, with their long-standing observations that the diameters of the disks varied with magnitude, even though Airy‟s discussion fully explained that variation. Even today, the conventional wisdom about stars seen in telescopes stands at odds to both historical observations and Airy‟s theory. However, an in- depth analysis of diffraction of light reveals that diffraction theory not only accounts for the general theme that brighter stars have larger disks than fainter stars, but also accounts for some of the finer points of detail in observations of telescopic disks of stars by visual astronomers. This in turn leads the authors to conclude that the historical observations of telescopic disks of stars discussed in this paper – including pioneering observations by 17th century astronomers Simon Marius, Galileo Galilei, Christian Huygens, and Johannes Hevelius – are valid and reliable data. This is despite that a casual review of these astronomers‟ work seems to suggest that their reports on stars are in disagreement. But it is the astronomers‟ interpretations of the observations that can be erroneous, due in large part to their ignorance of diffraction theory. The observations themselves should not be considered rhetorical enhancements, “thought observations” made for the sake of argument, or products of wishful thinking and crude instruments.
Historical observations of stars may even be of value to modern astronomers.
4 The physical bodies of stars: Simon Marius and Galileo Galilei March of 2010 marked the 400th anniversary of Galileo Galilei‟s publication of his Sidereus Nuncius – his first astronomical publication. The Nuncius includes a brief discussion of Galileo‟s observations of the stars in which he writes about a difference between the appearances of the planets and that of the fixed stars: …the fixed stars do not look to the naked eye bounded by a circular circumference, but rather like blazes of light, shooting out beams on all sides and very sparkling, and with a telescope they appear of the same shape as when they are viewed by simply looking at them [Galilei, Kepler, Carlos 1880: 40]….

Four years later, Simon Marius, in his Mundus Iovialis, reported that stars did show telescopic disks (Dreyer 1909: 191). In the Mundus Marius challenges Galileo‟s statements in the Nuncius –
…not only the planets but also all the more conspicuous fixed stars are discerned to be clearly round, and especially the bright stars of Canis Major, Minor, Orion, L

…(Full text truncated)…