I recently purchased a copy of One Hundred Leaves: A new annotated translation of the Hyakunin Isshu to try and improve my Japanese reading level. In addition to the English translation, each poem also includes the original Japanese and a rōmaji pronunciation guide.
The second poem in the collection is by Empress Jitō (持統天皇). In the book, the Japanese text is given as (emphasis mine):
春過ぎて
夏来にけらし
白妙の
衣ほすてふ
天の香具山
The corresponding rōmaji for てふ is given as chou. I understand that pronunciation can change over time, e.g. the particles は, へ, and を. But てふ ⟶ ちょう is a less obvious change.
Can someone please explain why てふ is pronounced ちょう?
Related info
I found this page which I think is trying to tell me that てふ is a contracted form of といふ, i.e. と[言]{い}う. Indeed, the English version of the poem on Wikipedia translates the てふ as "So they say". But I still don't understand how this relates to the pronunciation being ちょう.
I also looked for the answer on the Wikipedia page for classical Japanese language, which gives a bunch of examples of differences of pronunciation between modern and classical Japanese, but I don't understand the explanations well enough to determine if some rule or combination of rules listed on that page can explain てふ ⟶ ちょう (or was it ちょう ⟶ てふ?).
Finally, the kotobank.jp entry for ちょう〔てふ〕 mentions something about an [音変化]{おんへんか} and seems to indicate an intermediate step in the pronunciation change, i.e. ちょう→ちゅう→とう? Or maybe I am misunderstanding what the →ちゅう→とう on that page is trying to tell me.
