Is there such a thing in the academic world as "common knowledge"?
Some people will agree with, and some will oppose, the idea that some things are so obvious they don't need to be cited. Here're two essays on Wikipedia about whether one should cite that the sky is blue.

Wikipedia:You don't need to cite that the sky is blue


It's pedantic.
There is no need to verify statements that are patently obvious.
It leads to over-referencing where sentence flow is interrupted by excessive citations.
"The purpose of a citation is to guide the reader to external sources where the reader can verify the idea presented, not to prove to other editors the strength of the idea." 


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


Apparently simple facts may be disputed, e.g. the sky isn't always blue, it's black at night.
It's reassuring to readers, even if it's obvious.
Not everyone has the same "common knowledge". For example it may be common knowledge to Americans how many states there are in the US, but not to the rest of the world.


Suffice to say, there's no consensus on this. General guidelines then would be:


If the material is likely to be challenged, cite it.
If in doubt, cite it.