were you using auto focus?
does the D5100 have a live view type of function where you can digitally zoom in? you can pull a very accurate zoom on the moon that way.
If not, try manual focus, if your camera doesn't have a strong image stabilizer, it can be hard to get focus on the moon while the view is moving around lots.
also shooting in the F8-F11 area may help (not sure what aperture you shot at) it will allow you to focus easier.
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