Kepler answered immediately. He recalled having himself observed a sunspot in 1607 ‘of the size of a meagre flea’, which he had mistakenly assumed to be Mercury passing in front of the sun.6 He laughed at his mistake, then quoted reports of similar observations dating back to the days of Charlemagne; then gave his opinion that the spots were a kind of dross, due to the cooling of the sun in patches. Galileo delayed his answer for more than three months, and then claimed the priority of the discovery for himself. He alleged having observed sunspots for about eighteen months, and having shown
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