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He also compared the lengths of the tropical year ( the time it takes the Sun to return to an equinox ) and the sidereal year ( the time it takes the Sun to return to a fixed star ) , and found a slight discrepancy .
 tropical year, sidereal year
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 tropical year, sidereal year