50680
In 1679 , Newton returned to his work on celestial mechanics by considering gravitation and its effect on the orbits of planets with reference to Kepler 's laws of planetary motion .
 Newton, Kepler
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Newton, Kepler