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The astronomer 's mean tropical year , which is averaged over equinoxes and solstices , is currently 365.24219 days , slightly shorter than the average length of the year in most calendars , but the astronomer 's value changes over time , so John Herschel 's suggested correction to the Gregorian calendar may become unnecessary by the year 4000 .
 person: John Herschel
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
 Entities: John Herschel 
Answer:
 person: John Herschel