'At that moment' is more of a clinical, scientific statement of time, it's widely used. Whereas 'in that moment' refers not only to time, it also suggests involvement, participation. Also someone who is completely immersed in the situation with no other thoughts or worrys can be characterized as 'in the moment' .

The usual preposition is 'at', because you usually use 'at' for precise times, for example 'midnight', and times of the clock. However, you can sometimes use 'in that moment', suggesting that it is a longer period of time, or a time which seemed long to the person concerned. 'In that moment' is fine in the example you have given, because something happened during that time: the writer realised something about his Italian trip. If you're not sure, I'd advise you to use 'at', because it's more likely to be right. And remember that the preposition with 'the moment' is nearly always 'at' : at the moment.

Yes, I see. In fact I'd just checked Carmen's blog, and found the phrase in question, together with your discussion about it. And sorry to have assumed - for no good reason - that the writer was a man! ('his' Italian trip below). It's quite a subtle distinction, but 'in' is right here. And 'shed light' is a great phrase, too. Glad to have helped.

Yes Su.Ki. You shed light on our problem here: http://www.italki.com/entry/457171

Some light?