at a moment—at one point in time (out of many):
- He looked up from the desk, caught her at a moment when, against her will, tears had filled her eyes.
Note: The expression does not fully correlate in meaning with the phrase in a moment—
1. in a very short time:
- The maids are busy preparing your room but they will be done in a moment.
2. = at a moment:
- He forced the responsibility of two lives upon me, and, in a moment of indecision, I failed them.
at the moment—1. at present; just now:
- There are twenty residents in a converted private house, and at the moment many of them share a room.
2. at a specific point in time:
- That gentleman was really ruined, and that at the moment when he was about to attain his end.