89579
The Earth 's field has alternated between periods of " normal " polarity , in which the predominant direction of the field was the same as the present direction , and " reverse " polarity , in which it was the opposite .
 Earth
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:
 Earth