Wikipedia:You do need to cite that the sky is blue

It is sometimes felt that "obvious" statements, such as "the sky is blue", do not need citing. However, there are some reasons why you do need to cite the "obvious", such as that the sky is blue.

First of all, you do need citations in the "main" article, i.e., where the subject is the "obvious" statement or its major element. I.e., the statement "the sky is blue" must be footnoted in the article "Sky", especially in the section that discusses the color of the sky. Such references usually lead to more detailed knowledge.

Even the most obvious and simple assertion may need an explanation. The statement that the sky is blue needs explaining that it is due to Rayleigh scattering. Citing the sources that explain why it is blue would be valuable to some readers.

Readers come from different backgrounds and with different knowledges, cultures and experiences. It would be inappropriate to assume everyone's knowledge is the same.

Before making a statement, no matter how trivial or "obvious", make sure that it could not be misinterpreted or challenged.

Reasons

 * In certain contexts, such as in the sky article itself, and in the Rayleigh scattering article, there are reasons and physical mechanisms causing the sky to be blue that need explaining by expert sources.
 * Some editors may dispute apparently simple and obvious "facts". A statement that "the sky is blue" may be questioned because the sky frequently has a different colour—challengeable statements need sourcing.
 * A cite to a reliable source reassures people, even when the statement appears obvious.
 * The reason why the sky sometimes appears blue is technical, and requires an explanation that needs citing, if the technical discussion is at all relevant to the topic at hand.
 * Be aware of systemic bias and consider that people from other countries may not have the same perspective or common knowledge. Indeed, even the very concept of blue may vary between cultures and languages; in some cultures, the blue of the sky may be viewed, or at least spoken of, as a substantially different colour to that of ice or sea.
 * The colour of the sky might not be important enough to mention in the night article. If a source about night mentions the sky's colour, that verifies its relevance.
 * Be aware of the individual and unique nature of each of our readers.

Expert debate
The question is not whether readers can or can not be expected to have knowledge of a certain fact, but whether the fact in question is a relevant point of debate in the expert literature on the article topic.

If there is any doubt about the claim addressed in the relevant literature, you should cite it. If the relevant expert literature does not bother to address a point (e.g. because it falls under WP:FRINGE, and its mere mention, if only to debunk it, would lend it WP:UNDUE relevance), it is likely that the corresponding Wikipedia article shouldn't, either.

It's easier to find a citation than to argue over why it is not needed
If it really is common knowledge, it really isn't that difficult to source. For example, this source supports both the idea that the sky is blue, and that the blueness of the sky is common knowledge:
 * A field guide notes that "the blue sky is so commonplace that it is taken for granted".

One can go on to mention that the poet Robert Service says "while the blue sky bends above/You've got nearly all that matters". Songwriter Irving Berlin wrote of "Blue Skies smiling at me," airmen fly into the wild blue yonder. And one can, of course, cite Rayleigh's paper, "On the scattering of light by small particles," Philosophical Magazine 41, 275: pp. 447–451.

The "obvious" isn't always obvious
When a statement that you feel to be obvious is challenged, try to think of a person (such as a person in a foreign country) to whom the statement might not be obvious, or a situation in which your obvious statement might be wrong.

The sky actually appears to be blue less than half the time. Some conditions under which the sky may not appear blue:
 * During the night, the sky appears black. Without light from the sun creating Rayleigh scattering, the sky cannot be seen as blue, except in certain conditions when the moon is up.
 * Clouds can obscure the color of the sky. With cloudy weather overhead, the sky can be whitish, grey, or even black.
 * Natural disasters, such as forest fires, can lead to a haze over the sky, making it appear orange or red.
 * In the Bible, Jesus says to the Pharisees "When it is evening, ye say, It will be fair weather: for the sky is red".
 * Naturalist M. Minnaert notes that "At twilight, salmon reds, oranges, purples, white-yellows, and many shades of blue can be seen."
 * Songwriter Oscar Hammerstein wrote of "when the sky is a bright canary yellow."
 * On other planets, the sky is almost never blue.
 * Blind people can't verify the color of the sky. If the respected source is present, the statement could be more trusted.
 * People with tritanopia (commonly known as blue-yellow color blindness) may see the sky as green instead of blue due to absence of blue cones in their eyes.