Princess Celestia does not like her sun. She likes pancakes however. Pancakes are perfect. Her sun is not.
Therefore, she does not like her sun. Q.E.D.
"Sister, are you alright? It has been a few nights since that that morning letter, and yet you still continue to wear down a circle in the carpet each and all night. I hope my neglect for your most delicious pancakes were not the cause of this. If it were-"
"No." How could she still like her sun anymore? Especially since her student, no, ex-student somehow managed to find fault within her perfection that was the sun.
It was the first letter she had sent ever since her coronation, and it was purely mocking her of her sun. The literal representation of who she is. Was.
"No, it is not your fault Luna. Just a mere conundrum I face from Twilight."
"You refer to Princess Twilight Sparkle? But what has she done to result in your disturbed manner?"
Celestia plants her hooves down on the luscious carpet. A carpet which is scorched from the morning's incident a few days ago. She passes over the letter which had been orbiting around her head to Luna. She stands there, eyeing her sister as she awaits for the response.
"I... see," whispered Luna.
Celestia raises her eyebrow in discontent.
"You just 'see'? Do you not realise"- every word spat with icy resent -"what the implications of this is? I am Princess Celestia, the alicorn who controls the sun's path across the sky. The dictionary probably even has a portrait of my face under the literal definition of the word 'Sun'."-
She magics a thick dictionary into existence and flicks to under the letter 'S'
-"Well would you just look at that! It actually does! The sun is as a part of me as I am a part of it. And to think that somehow I have gotten it wrong. Somehow misjudged it. For at least a thousand years no less!"
Her sister blinks at her.
"You know what? Maybe the ponies are right. Maybe the sun is an orange. Maybe I have just been delusional for all this time. Maybe- Oof!" Celestia is struck by a round, colourful piece of fruit before she could continue further.
"Dear sister, was the fact that you made a simple mistake been the cause of your troubles all this time? And they call me the emotional archaic. I am severely disappointed in your actions Celestia. Princess Twilight here has found evidence for a new threat on the horizon. Do you not wish to contact her to discuss further in such matters?"
Celestia isn't focusing on Luna however. She merely observes the orange that had made quite a terrible first impression with her face.
"Oh, we will discuss further on these matters indeed. She has found a fault of mine, so I shall do the same to her. It is only polite, after all to do unto others as they did unto you"
Princess Luna frowned. "What do you mean by this? Were you listening- Urk!"
A round, colourful object makes acquaintances with her face.
"Now, now, let me see. What terrible, horrendous mistakes has she made so far?" Celestia calls forth a crystal journal with her magics. A book which contains a record of her former student's life ever since she started mentoring her. The times when she has succeeded in casting a new advanced spell. The times when she learned a friendship lesson.
The times when she has made an absolute mockery of herself.
Flicking through the book, she scans the pages for nuggets of retribution. The 'Want it, Need it' incident? Her advances towards a certain stallion in the human world? Or perhaps her general neurotic and antisocial behaviour before being sent to Ponyville.
"I have got it Luna! Quick, bring me quill and parchment! We have a letter to write back!"
Luna tenderly rubs her face with a hoof as her glares throw daggers at her sister. Muttering darkly under her breath, she closes her eyes and magics the items as per Celestia's request, albeit requiring a moment's concentration due to the throbbing of her head. She opens her eyes to find her sister already rattling off her message to a parchment enchanted with an ascribing spell.
Screaming incoherent curses in her mind, she struts out of the room. She may as well start her duties in the dream realm now that most ponies would be asleep.
Princess Twilight Sparkle wakes up. However, before she fully raises her head, she remembers a critical detail about the location of her slumber and teleports out onto open area. She waits for the momentary dizziness to pass as she sighs in relief. For she managed to avoid having her forehead cut from that obscene shard of crystal sticking out of the wall, right above her bed.
A shame it had taken her a fortnight to figure it out.
Shaking her head clear of the remaining fogginess, she walks over to the bathroom. While she performs her simple morning routine, she reminisces of the past few days. The time when the entirety of Canterlot's pretentious nobles flocked around not unlike headless chickens while the Guards tried to calm them down. She thoroughly enjoyed watching the events unfold along with Celestia.
Yet, something felt off on the day. Twilight brushes her mane as she frowns. Princess Celestia was not acting like usual self that day.
Maybe it was the buttered popcorn. Yes, that was it. She knew she shouldn't have used orange trimmings to flavour it.
Twilight canters into the dining hall of her castle, following the smell of freshly cooked breakfast. As she makes herself comfortable, Spike walks out of the kitchen, wearing a frilly apron and an oddly-fluffy and white top hat. He balances a semi-circular metallic dome on his arm while the other is behind his back. Quite an impractical and inefficient serving, but regardless...
He coughs eloquently and speaks in a formal air, "Morning, Count Twilight Monsparkle."
"Spike, what's goin-"
He hastily interrupts her. "I present to you an exotic dish for the morning's breakfast; Surprise d'Orange Glacée" He lifts off the silver dome, revealing the meal inside. Twilight frowns.
"It's just a pair of or-"
"And a very important letter for your eyes only, which will take you enough time to read in order for me to go to Sugarcube Corner and get you something that would actually count as breakfast as I totally didn't let Pinkie raid through our stores in exchange for a handful of gems. Got to go now, bye!"
Twilight blinks, but soon disregards the matter and opens the letter which had been slapped into her face.
From Princess Celestia? Oh, it must be about the prophecy!
Twilight squeaks in a god-almighty pitch that would rival Fluttershy's squee as she reads the message.
Although she leaves the oranges be. Prefers pancakes in all honesty.
Dear Princess Twilight Sparkle
I am so glad that you had sent a letter to me. It was simply divine that we could still continue this tradition of ours alive even with your Princess duties running in the way. Surely you must be extremely busy in managing diplomacy with the various nations and treating them with the magic of friendship, instead of wasting your time and effort merely researching an already debated topic, for an extended period of time.
I can assure you that history's finest scholars have poured their entire lives into interpreting these predictions, and they all pointed towards my sister's return a few years ago.
Even then you continue with your own theory, which somehow makes sense yet still points towards me making a thousand-year-and-counting running mistake. I... thank you from the bottom of my heart for noticing my errors, and I will be sure to fix it in the soonest moment. It is not as if the sun is my entire livelihood, a reflection of my perfection as being a Alicorn who literally appears under the definition of the word.
Look it up if you do not believe me.
Now, onto your interesting interpretation.
The theory which you present would have held water, were it not for the fact that the line "the stars will aid in her escape" discredits your entire argument. I believe I speak for both of us when I say that the stars have been observed converging on the moon, allowing Nightmare Moon to gain the power required to make the shift onto Equestrian land.
Still, a prophecy must not be literally taken as truth, so the stars may have meant differently. I completely agree with the fact that prophecy did talk about the Nightmare, so on the account of your theory being true and literally everypony including myself misjudging it - which I should remind you has less chance of happening than my sister actually eating my pancakes instead of wearing it as makeup - my best guess would be that the Nightmare returns to inhabit a different a pony as a host. In this case, the mention of stars would refer to unicorns of a high talent in magic, with a cutie mark related to celestial objects to boot.
Ring any bells?
Actually, I was meaning to ask you a personal question along with this. Consider it an... eye for an eye for the internal crisis you have caused me. I would have regressed into a full soliloquy were it not for my sister.
Do you remember a mare named, 'Moon Dancer?' You know, the pony which you abandoned on her birthday party just to read on a few books. Granted, your course of action led to the reformation of Princess Luna, but ever since then, have you given any consideration to her, or the rest of your Canterlot friends in that matter? The last I've heard of her is that she had become an alcoholic recluse who is filled with spitting jealousy at a certain lavender unicorn.
And to think that as the Princess of Friendship you have failed in one of your first friends...
However, what makes this all the more important is the fact that she fits quite perfectly in the criteria as a host for Nightmare Moon.
Unicorn with an aptitude for magic? Check.
Cutie Mark related to some celestial object? Check.
Filled with resentment and jealousy towards somepony, especially right after the second defeat of Nightmare Moon? Check.Seems as if we have found our culprit, but you don't have to worry about that. I have released this information to the Higher Magics Order, and I'm sure somepony there is already testing the area for dark arcanical energies. I'll make sure to send you a copy of their reports once they have done with the analysis.
So in any case, rest easy, my dear Twilight. You shouldn't worry about anypony else. Do your duties and we'll do ours.
I look forward talking with you soon on matters other than our roles as princesses.
- Princess Celestia
P.S. If you didn't know already, do not put orange rinds on popcorn. It's a fate worse than death.
Spike runs in distraught with a tinge of fear just as Twilight finishes the letter.
"I'm so, so sorry Twilight. I've been to Sugarcube Corner and all they had was orange-related items and I don't know why they have that, and Pinkie said that her emergency stash of emergency breakfast backup food had just run out, so I thought I might as well get something but I forgot to bring enough bits to buy anything other than more oranges so I'm so sorry and please don't use your princess powers to banish me to Tar- Twilight?" Spike notices Twilight, evidently out of shift with reality from the letter she is reading.
Without glancing up, Twilight asks with a befuddled frown on her face, "Spike, Princess Celestia said something about a mare named 'Moon Dancer'."
Spike takes a few moments to compose himself and consider the question before answering, "Oh, Moon Dancer? She was one of your first friends." After receiving a confused look from Twilight, he continues on, "You do remember who she is, right?"
Twilight places the letter away on the table. She ruffles her chin with her hoof for a few moments. After which she merely shrugs her shoulders at Spike.
"Doesn't ring a bell."