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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 .
 person: Giovanni Domenico Cassini; person: Christiaan Huygens
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 person: Giovanni Domenico Cassini; person: Christiaan Huygens