Why 永遠 sometimes is written as えいえん and とわ?

I have recently listened to a music which has this part :

色々見てきたけれどこの瞳は永遠に きらり
永遠に きらり

The main thing is that I have always heard that this kanji  永遠 it's written as えいえん, however in this song he sings とわ in the   永遠 kanji.

Please you guys gods of japanese language, can anyone help me with this question?

Thanks in advance!

Both えいえん and とわ are correct readings for 永遠.  えいえん is common in everyday usage, while とわ is used in more poetic or classical settings.

Also of note is that とわ acts as a special reading for both kanji 永遠, similar to how あした is for 明日.  ie, you can't read 永 as と, or 遠 as わ, but when they are put together 永遠 you can get the reading とわ.  The reason for this is that when Kanji was first introduced to Japan from China they would have matched the Japanese word とわ to the meaning of the Chinese Kanji 永遠.

flint wrote today at 8:31 PM

Both えいえん and とわ are correct readings for 永遠.  えいえん is common in everyday usage, while とわ is used in more poetic or classical settings.

Also of note is that とわ acts as a special reading for both kanji 永遠, similar to how あした is for 明日.  ie, you can't read 永 as と, or 遠 as わ, but when they are put together 永遠 you can get the reading とわ.  The reason for this is that when Kanji was first introduced to Japan from China they would have matched the Japanese word とわ to the meaning of the Chinese Kanji 永遠.

Nice explanation! thanks flint