I always thought it was a moon.
As for the Yin Yang thing...both are probably correct.
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A very primitive use of the Chinese characters for yin and yang is claimed to date back to the fourteenth century BC. It is thought that initially the character for yin described the night and yang the day. It seems that Chinese philosophers recognized that humans where influenced by three powerful cycles, those of the day, moon and year. It also is apparent that our response to these cycles has certain similarities. So we might notice similarities with the night, new moon and winter whilst sensing a similar response to the day, full moon and summer. Yin then represents the night, new moon, winter part of the cycle and yang the day,
full moon and summer.
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Nothing is absolute with Yin and Yang. The designation of something as Yin or Yang is always relative to some other thing. For example,
day is Yang, yet within every day is a Yang part—the early morning, and a Yin part—late day, as it begins to turn to night, which is Yin. - See more at:
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Otsutsuki is a clan heavily connected with the moon, so it'd make sense.
New moon/Shrouded in shadow-Yin=Indra
Full moon/Shining in light-Yang=Ashura