"Sunset on Mars is blue."
Is this statement grammatically correct. Of course there is also the concern if accuracy/logic overrides grammar.
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There's grammar, there's semantics, and there's the observable world.
Only grammar and semantics are important to communicate in a language.
If I break grammar (The dogs is work hardly) it will be hard or impossible to understand what I mean.
If I break semantics (Colorless green ideas sleep furiously - Chomsky) it will be hard to assign any meaning to the sentence at all.
But when I make a statement that is simply false, I can do so in a way that my audience understands me perfectly well, so from a language point of view, the sentence can be fine.
If your suggestion that false statements are grammatically incorrect were true, one could wonder how children, who learn the grammar and semantics of their native language in a very effective way, would be able to tell a lie. After all, they learn that grammatically wrong sentences may not be understood at all, so if a lie would be ungrammatical, nobody would understand what they weer saying while telling a lie.