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Ratio between the intensity of central umbra (darkest part of sunspot) and intensity of lighter surrounding penumbra

Value	0.108 unitless
Organism	Biosphere
Reference	Keller & Friedli, Visibility limit of naked-eye sunspots, Q. J. R. astr. Soc (1992) 33, 83-89, link p.89 3rd paragraph
Method	P.88 1st-3rd paragraphs: link
Comments	P.88 5th paragraph to p.89 3rd paragraph: link bottom paragraph in link: "This means that the intensity of the umbra is per unit area 9.25 times lower than the intensity of the penumbra."
Entered by	Uri M
ID	114252