I was studying planet names and noticed some similarities. Are the days of the week named after the planets?
Yes, they are, and it comes from Western Influence.
日曜, 月曜, 火曜, 水曜, 木曜, 金曜, 土曜 are Classical Chinese names for the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn, respectively.
English names for the days of the week are mostly Germanic names for the same planets. Sunday, Monday, and Saturday are obvious, but we have
- Tuesday (Norse: Tiw for Mars)
- Wednesday (Norse: Odin; Old English: Wōden for Mercury)
- Thursday (Norse: Thor for Jupiter)
- Friday (Norse: Frigg for Venus)
Chinese no longer uses this name scheme, but Korean and Japanese still use this system.
this has nothing to do with japanese, but you can find the planets also in the weekdays of other languages and interestingly, it is not always the same ones. i found that with spanish (or any other romanic language) and english together you can derive all planets:
monday - lunes: luna = moon
tuesday - martes -> mars
wednesday - miércoles -> mercury
thursday - jueves -> jupiter
friday - viernes -> venus
saturday -> saturn (sabado in spanish)
sunday -> sun (domingo in spanish)