On the Telescopic Disks of Stars: A Review and Analysis of Stellar Observations from the Early Seventeenth through the Middle Nineteenth Centuries

Abstract

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

Contra Galileo: Riccioli’s “Coriolis-Force” Argument on the Earth’s Diurnal Rotation

“…37 I have discussed elsewhere the challenges that telescopic observations of stars presented to the Copernican theory, as Simon Marius (1570-1624) noted in 1614, 38 and the importance that Riccioli attached to them. 39 Briefly, stars as seen through early small-aperture telescopes appeared as well-defined disks, as the astronomer John Herschel illustrated in 1848 (figure 3), 40 whose spurious nature was not recognized, 41 and hence were interpreted as their physical sizes. Thus, the Copernican assumption that stars lay at vast distances from the Earth (as they must to explain the absence of observable stellar parallax) came with a cost: the farther away the stars, the larger they had to be.…”

Sunspot numbers based on historic records in the 1610s: Early telescopic observations by Simon Marius and others

“…Marius and other observers at that time saw stars as round circles through their (very small) telescopes, today known as Airy disks, but thought that they had resolved the stars; by assuming that stars have the same diameter as the Sun, they could then www.an-journal.org estimate their presumable (but far too small) distances; from the non-detection of their parallaxes they rejected the heliocentric model and preferred the geo-heliocentric model. See Graney (2009Graney ( , 2010Graney ( , 2015 and Graney & Grayson (2011) for details.…”

The Starry Universe of Johannes Kepler

“…(These disks were not then understood to be spurious products of the diffraction of light waves -the "Airy disk" phenomenon. 10 ) The telescopic disks were smaller than what Brahe had measured, and this was attributed to the telescope stripping away glare and revealing the true bodies of the fixed stars, much as Hooke described above (and just as the telescope revealed the true bodies of wandering stars, or planets 11 ). Riccioli showed that the telescopically measured star sizes, combined with the increased ability to detect parallax that the telescope provided, still yielded giant stars.…”