Visual limits and the visibility of infinitesimals

Marco Piccolino and Nicholas J. Wade

in Galileo's Visions

Published in print December 2013 | ISBN: 9780199554355
Published online January 2014 | e-ISBN: 9780191766978 | DOI:
Visual limits and the visibility of infinitesimals

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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

Chapter.  15863 words.  Illustrated.

Subjects: Cognitive Psychology ; Neuroscience

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