you are correct that works but instantané is the adjective form. "C'est un instant" is talking about the noun not the adjective. The adjective means "happening or coming immediately" or "urgent/pressing". The noun means "a precise moment in time" or "a very short space of time". So basically with the duo phrase you are saying "it is a precise moment in time" or "it is a very short space of time". It's just a nice short way of saying it that gets the same idea across
In speaking, an instant is very very quick, and a moment is quick. "I'll be there in an instant" means a matter of seconds. "I'll be there in a moment" means I'll be there very soon. Maybe a minute or two. You could use either interchangeably though. No one will care.
Technically, an instant is an infinitesimally small measure of time. So if you went somewhere instantly you would have to teleport. According to wikipedia a moment is an old measurement for 1/40th of an hour. But no one uses it like that anymore. It just means "a short length of time".
I am neither a French nor English native, but a German, so the following might have some mistakes, but in my French class "un moment" and "un instant" was discussed intensively and here is what I learned: My French teacher (a native French) told us, that in French "un moment" and "un instant" are not interchangeable, because "un moment" means a longer period of time which can extend to several month and would translate to currently, while "un instant" means a very short period and would rather be translated to "a moment." If you meet someone and you ask him: What are you currently doing?in French he might answer: Pour le moment je travail à mi-temps, mais ... - Currently I'm working part-time, but... While on a phone call, if you put down the receiver for a moment you would say: Un instant. - Just a moment. Telling him "un moment" in such a case would be inappropriate.
Interesting. I have heard people use instant a lot more here. E.g. at the pub if someone asks if a chair is taken I hear "pas pour l'instant" in place of "not at the moment".
I think "at the moment" works the same in english as in french though. "in a moment" and "at the moment" are a little bit different, at the moment just means currently like you mention above.
An instant and an instance aren't the same thing. An instant is a precise or very short moment in time. An instance is an example or occurrence of something, or one thing in a series. Say I point to my watch and when the second hand ticks once, I say, "That was an instance (an example) of an instant (a short moment of time)." Now say you ask me how much time had just passed. I might reply, "It was a mere instant, in this instance (case)." For what it's worth, the French for "an instance" is «un cas» (one of a series) or «un exemple» (an example of something). Think of the English "case" and "example."
I put " This is pressing" but it wasn't accepted. "It is an instant" is not anything I've heard in my 35 years... unless... maybe someone's asking you what type of grit is in your bowl (referring to just one grit, of course, b/c that's the only way this would make sense), and you respond, "it is an instant (grit)."