I know it's just fan art mostly, and it's not too big a deal, but I can't help but notice there seems to be this general idea that Luna spent 1000 years on the moon, all lonely and sad. Well, this is a little bit wrong. Before Luna was sent to the moon, she 'transformed into a wicked mare of darkness'. She actually transformed into Nightmare Moon first. Celestia was like, 'woah, that aint cool', so off to the moon she went. Luna never went to the moon. Nightmare Moon did. So, as beautiful as some of them are, all those fan pictures of Luna on the moon are by that logic wrong. Now, for the actual banishing itself. It clearly states that Celestia 'locked her inside the moon', not on the moon. To me, this conjurs up the image of a prison or dungeon. I'm not sure, but it sounds cool. Either way, she was never on the moon. If she was, she would have just spent 1000 years giving it aeiou aieou John Madden UUUUUUUUU Holla holla get dollar Football aeiou. Not cool. So really, a more accurate description of what happened would be one of Nightmare moon being in some sort of moon prison under the surface, spending 1000 years being angry and probably shouting a lot. Not one of Luna sitting on a rock looking sad. Although as I said, some of those pictures are very nice. I'm just spouting ideas, feel free to ignore.
I see your point, it is more logical for it to happen the way you describe. I guess Nightmare would have a lot of time to be angry up there, but being in the moon or on top of the moon doesn't really matter. She couldn't get off the thing until a 1000 years anyway. I like the thought you put into it though. The Luna pictures are always nice, got a few as wallpaper for my phone.
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I'm not even sure if Nightmare Moon spent all the time on the moon at full conscience and aware of the time passing. The mare we see in the show doesn't look like one who had just returned from experiencing 1000 years of (presumable) solitude in some dungeon in the moon. I rather think that Celestia somehow merged her into the moon itself, and that she didn't take notice of the actual time passing. It might have been a sort of stasis. One thing has me wonder, though, and that is the legend of her escape. Celestia actually banished the abomination that had become (or gotten hold) of her sister into the moon permanently, at least that was her intention. However, the legend says that one thousand years after her banning, the stars (the four stars which seemed to always surround the moon) would aid her escape. The banning story was considered a legend, yet it turned out to be recorded history. At least Celestia was present at this point to tell what happened. But how did the information about Nightmare Moon's return (and the exact date) make it into that old legend book? Did Nightmare Moon prepare and then announce her own liberation as her last deeds before she went into the moon? And who heard about it if not Celestia who seems utterly uninterested in taking precautions for the Summer Sun Celebration? Or did Celestia simply forget about Nightmare Moon's announced escape or never believe in it in the first place?
I'd say that Celestia never intended to have Luna trapped on the moon forever. I think that she was fully aware of the impending escape of Nightmare Moon, but as she hoped (or even knew) that there were now ponies able to break NMM's grip on Luna, she just let it happen. In fact, the date of NMM's return being the Summer Sun Festival might imply that it was Celestia herself who made the escape possible (don't forget that after all, the sun is a star, too).
Nightmare Moon was conscience, at least partially. When she returns on the night of the Summer Sun Celebration, she says, "Oh, my beloved subjects! It's been so long!" Which implies that she somehow knows time has passed, likely an extended period of time. Also, she doesn't seem at all confused 'waking up' in Equestria 1000 years later. You know how much stuff has changed for us in the last 1000 years? It would likely be a totally different world, and yet NMM is seemingly unphased. Although, she does head to the castle ruins in the Everfree Forest, which was likely the former castle where she and Celestia ruled, before Canterlot was built. Why did NMM arrive in Ponyville rather than in Canterlot? How did she know that's where Celestia would be? Also, where the heck WAS Celestia during all this? And gosh darnit, what do the four stars that aided in her escape represent?