New answers tagged katakana
Before the modern kana usage (現代仮名遣い) has permeated through the nation, it was very common for katakana ツ to be used after a numeral kanji. Japanese traditional numerals are: 1 - ひとつ 2 - ふたつ 3 - みつ ／ みっつ 4 - よつ ／ よっつ 5 - いつつ 6 - むつ ／ むっつ 7 - ななつ 8 - やつ ／ やっつ 9 - ここのつ 10 - とお And today, these are written ...
Simply the stylistic choice to use a variant pronunciation. [tju], [tʃu], and [tu] are all allophones in English. Japanese typically uses [t͡ɕu] (close to [tʃu]) for Neptune because it's the closest match to the native phonological "rules". This atypical (much more so than トゥ) form is just "artistic freedom" IMO.
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