Why are the days of the week in Japanese named after classical elements?

A random thought stumbled into my mind the other day when I realized that Sunday matched the Japanese word for Sunday is “日曜日”. The “日” part means “sun”. At first I thought it was just a coincidence, then I realized that Monday represented the moon, and in Japanese that’s “月曜日” and the “月” part means moon. Something is clearly going on here. Since the idea of seven days in a week doesn’t originate from Asia, the Japanese days of the week probably just followed western convention, which makes sense. Now when it comes to the English days of the week, the names were originally named by the Romans with the names of the sun, the moon, and the five known planets at the time [1].

Now on the other side of the world, the Chinese and the Japanese gave names to these planets using their classical elements (the Chinese and Japanese names are the same). They gave the celestial bodies these names:

So now, when the Japanese went to adopt the western calendar, they took the base meanings of the days of the weeks by their planets, and used their planet names in their place. So Sunday (from the sun) became “日曜日”, Monday (from the moon) became “月曜日” and so forth. Like this:

Cool, huh?