DescriptionDiagram from Galileo's Third Letter on Sunspots on differential speed of sunspots 02.jpg
English: If a sunspot moves along the Sun's equator from B to A, on the surface of the sun, its transit time is longer than that of another sunspot moving from L to D. However if the sunspots are not on the surface but above it, at C and E rather than B and L, the differential speed of their transit is reduced. (CB equals BA, but EL does not equal LD). The closer the sunspots are to the surface, the more their relative speeds can vary; the further they are from the surface, the less difference in speed will be observed.
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Own work, from https://archive.org/details/bub_gb_VFhEYHsLn3EC : Istoria e dimostrationi intorno alle macchie solari e loro accidenti comprese in tre lettere scritte all'illustriss. sig. Marco Velseri linceo duumuiro d'Augusta ... dal signor Galileo Galilei linceo ... Si aggiungono nel fine le lettere, e disquisitioni del finto Apelle, by Galileo Galilei 1655. Publisher: per gli HH. del Dozza. Book from National Central Library of Rome.
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