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

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


My question is inspired by this book's title:

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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".

  • 2
    One is properly punctuated, the other is not. "why the sky is blue" would be more typical of an exclamation, and properly punctuated "why, the sky is blue!" It's not the proper sentence structure for a question.
    – Misneac
    Nov 14, 2015 at 15:30
  • 6
    Asking "Why the sky is blue?" typically announces to your audience that you're not a native speaker. A more complete explanation of why nns often get this wrong (and how to avoid such errors) would be better addressed on English Language Learners Nov 14, 2015 at 15:33
  • 1
    This has nothing to do with why; the same problem arises with any question. There is subject-auxiliary inversion in normal questions, whether they have wh-words or not. Nov 14, 2015 at 15:39
  • 1
    @Misneac, I was aware that "Why is ... " probably is the correct form of asking a question, but I doubt it when I saw this book's title.
    – HadiRj
    Nov 14, 2015 at 19:15
  • 1
    So why did you add a question mark to the title of the book, @HadiRj?
    – TRiG
    Nov 15, 2015 at 0:45

3 Answers 3


"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.


It's very simple, you are adding the question mark. That is why is reads funny to you and the other posters.

The book title is NOT a question. It is a statement.

The presumed question is, "Why is the sky blue?" Because David Richerby understands how Rayleigh scattering works, he could write a book explaining it to you. He might call that book, "Why the sky is blue."

A similar phrase is used in "I know why the caged bird sings" by Maya Angelou. It could have been called "Why the caged bird sings."

  • 1
    For what it's worth, the sky is blue because of Rayleigh scattering, not refraction. Nov 15, 2015 at 11:18
  • Ok, I fixed it, David.
    – Engineer
    Nov 15, 2015 at 19:49
  • I'm flattered. I suspect that wouldn't be a very good book. :-) Nov 15, 2015 at 20:00

It's all about 'subject-verb inversion' which is something that happens in English when certain kinds of element occupy initial position in the clause:

[1] The sky is blue.

[2] Why is the sky blue?

[3] *Why the sky is blue?

Ex [1] is a simple declarative clause with "the sky" as subject and the auxiliary "is" as the verb. Inserting the question word "why" at the beginning triggers subject-auxiliary inversion to give the interrogative [2]. Likewise in [3] "why" has been inserted at the beginning, but this time no inversion has occurred so the clause becomes unacceptable in Standard English.

  • Why was my answer changed by (apparently) Mitch? Is this normal practice, or what?
    – BillJ
    Nov 14, 2015 at 21:07
  • I didn't change it, but if you like, you can roll that back through the edit history. IMHO people shouldn't just randomly edit each other's posts like that, but some 2k+ users evidently feel differently.
    – Kevin
    Nov 14, 2015 at 23:24
  • 2
    @sumelic It's not "random" but one of the reasons for rejecting edits is "Clearly conflicts with the author's intent." My feeling is that the edit in question, which was essentially putting words into BillJ's mouth, would have been rejected for that reason, even though I'm sure it was made in good faith. Nov 15, 2015 at 11:21
  • @Mari-Lou: Edits which get rolled back are unhelpful in my book. I would have made a comment instead.
    – Kevin
    Nov 15, 2015 at 15:34
  • 1
    I undid the edit because I thought it was potentially misleading to say that inversion does not occur in relative clauses like “x is the reason why the sky is blue”. I think it would be better to say that it’s subordinate clauses in general that don’t display inversion. Relative clauses are a subset of subordinate clause – other members, like content clauses, also do not display inversion, e.g. “It’s obvious why the sky is blue”.
    – BillJ
    Nov 15, 2015 at 17:57

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