83631
Having compiled a list of 24 comet observations , he calculated that the orbital elements of a second comet that had appeared in 1682 were nearly the same as those of two comets that had appeared in 1531 ( observed by Petrus Apianus ) and 1607 ( observed by Johannes Kepler ) .
 Petrus Apianus, Johannes Kepler
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:
 Petrus Apianus, Johannes Kepler