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Galileo's VisionsPiercing the spheres of the heavens by eye and mind$
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Marco Piccolino and Nicholas J. Wade

Print publication date: 2013

Print ISBN-13: 9780199554355

Published to Oxford Scholarship Online: January 2014

DOI: 10.1093/acprof:oso/9780199554355.001.0001

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Visual limits and the visibility of infinitesimals

Visual limits and the visibility of infinitesimals

(p.119) Chapter 8 Visual limits and the visibility of infinitesimals
Galileo's Visions

Marco Piccolino

Nicholas J. Wade

Oxford University Press

Galileo addressed the visual appearance of small luminous bodies with particular relation to measures of the angular size of stars. By simply using a cord to find the smallest angle capable of occulting visibility of a given star he was able to show that the sizes of prominent stars like Sirius or Vega were 5 seconds of arc or less. These values were much smaller than the 2–3 minutes of arc generally supposed since antiquity and confirmed by Tycho Brahe. For reasons discussed in the chapter, Galileo’s conclusion was of paramount importance with relation to discussions on the credibility of the Copernican system, particularly stimulated by a booklet co-authored by Galileo’s old adversary Christoph Scheiner. The important point in the star-occultation measure was Galileo’s intuition that the apparent size of the star was the result of phenomena largely occurring within the eye. This intuition is discussed with reference to modern knowledge of the physical and physiological constraints of vision.

Keywords:   limits of vision, point light sources, diffraction, pupil, visual optics and physiology

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