Moment: “very brief portion of time, instant, moment, minute; importance, weight, value”, and directly from Latin ‘momentum’ movement, motion; moving power; alteration, change”.
The sense of "notable importance, 'weight,' value, consequence" is attested in English from 1520s. Meaning "opportunity" (as in seize the moment) is from 1781.
In careful use, a moment has duration, an instant does not.
-“We are constant moments”.