The Solar System and Names for Days
For some time following adoption of a lunar calendar, Japan used Chinese sexagenary cycles for naming days and years. However, there is evidence that from 807, a seven day week with names related to the “planets” had found its way to Japan.
The seven “sei” (solar system bodies including the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn) were as important to early calendar scholars in Japan as they were anywhere else, regardless of the inadequacy of accurate methods for determining their motion.
The names come from the five visible planets, which in turn are named after the five Chinese elements (wood, fire, earth, metal, water), and from the moon and sun (yin and yang).
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