meaning - "Why the Sky is Blue"

"Why is the sky blue?" vs "Why the sky is blue?"

It seems both sentences are correct, is there any difference in meaning?

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Addendum

My question is inspired by this book's title:

Edit:

I added question mark to the "Why the sky is blue" based on the wrong assumption that if a sentence begins with "why" it's a question".

Answer

"Why is the sky blue?" has the grammatical structure of a question, and cannot be interpreted in any other way.

"Why the sky is blue" has the grammatical structure of a phrase standing in for a noun; it could be replaced by "the reason for the sky's blueness" or "the reason the sky is blue". E.g. one can say "Why the sky is blue is a fascinating question". It can also be interpreted as a question, but technically this is not grammatically correct.