Have you ever watched a live soccer game on TV? Have you ever watched a soccer game replay, even without knowing what the final result was? The difference between the feeling produced by the live game and the feeling produced by the replay is the exact feeling that makes the reflection about an instant possible: the world, as we know it, happens in the past, although we live it in the present.
I started wondering about an instant's content. An instant is something that doesn't last long, that's for sure. But, when you come to think of it, very few things do, and, when they do, we usually lose interest in them. It is as if humans were programmed to be attracted by newness. Is there a universal standard for an instant's duration? If there is one, then it must have been set according to human perception capabilities. But, even if there's a psychological standard—how often does the working memory buffer refreshes, for instance—a more significant standard should have to do with an instant's importance (determined by its content's newness).
So the matter in question, then, isn't how many frames per second a human being can "capture," but why, within those biological boundaries (if there are any), some instants are captured and some others are discarded. I think that the most fundamental property of an instant is the time when it happens, its temporal context. When an instant's content becomes too relevant, that instant turns into a moment. Only then, a moment can last several instants. "One of the best moments in my life," for example, is a phrase that may contain various instants, each of which has a meaning of its own, but only because of a bigger importance frame: a moment.
A moment, nevertheless, refers not only to an important instant, it can also allude to the time comprehended between the present (now, as you read it) and a future, significant event. "It will come in just a moment," for example, is a phrase that tells how far in time we are form a desired (or at least important) future.
An instant, then, is an arbitrary unit that measures the minimum amount of time we can access. A moment, on the other hand, can either be an arbitrary unit to fragment the continuum of life into significant events, or an arbitrary sign to generate expectations and turn on the mental recorder.
Often, we have to wait just a moment for an important moment to come. Sometimes, however, that moment never comes. So, what's in a moment? An instant? A meaning? Let's wait and see, only time will tell.