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and discovered that the martian ice caps , first observed by Giovanni Domenico Cassini ( 1666 ) and Christiaan Huygens ( 1672 ) , changed size with that planet 's seasons .
 Giovanni Domenico Cassini, Christiaan Huygens
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Giovanni Domenico Cassini, Christiaan Huygens