Princesses Celestia and Luna are widely accepted as two of the most mysterious characters in My Little Pony Friendship is Magic. Their power is, in many senses, out of this world. They move the heavens on a daily basis and draw power from the sun and moon. Many have even ventured to call the two sisters goddesses.
Being the only two Alicorns in Equestria for the first fifty episodes of the series, the two sisters are undoubtedly beings on a greater tier than the average pony. However, the actual loftiness of this tier is up for debate. For a long time, I have had my own answer to the goddess question, and I now intend to put it forth.
I believe, in the context of the fantastic world of Equestria, Celestia and Luna do belong to a particular breed of mythical beings. But goddesses? Let us see...
On the Origin of Two Ponies
In the beginning...
Or, rather, shortly after the founding of the nation, the two sisters were made princesses of Equestria. This is an event we know outstandingly little about. Almost all of the information about this event comes from The Journal of the Two Sisters. However, I believe this book provides enough for one to gain an idea of the sisters' early lives.
But first, we must note that the first chapter of the book is remarkably vague. No setting is described at all. Star Swirl the Bearded and his coterie "approached" the two sisters, and that was it. Where and how are unanswered.
This means a few things, the most important of which is the strength of the sisters' relationship. Celestia and Luna were together before the founding of Equestria. Is there anything strange about this? Actually, there is. The two sisters, even though they lacked their cutie marks, were distinctly adults when they accepted the throne. Most siblings do not stick together far into adulthood. Look at Pinkie Pie, Maud Pie, and Limestone Pie and Marble Pie. These ponies went their separate ways to pursue their own interests. They parted to live their own lives.
The question of why the royal sisters would remain in each other's company well into adulthood naturally follows. The most basic and unpretentious answer is responsibility. One sister, likely the older one, felt responsible for the other's well-being. This responsibility does not seem to have been shared with anyone else, for neither sister mentions any parent or guardian in the journal. They do not mention any home either. If they ever knew their parents at all, which is likely in light of their knowledge of Alicorn magic, then they must have been separated from them at a young age.
Accordingly, this is the image that arises from this introduction: Celestia and Luna are two Alicorn sisters who live completely alone, taking care of each other because no one else will and possessing no other obligations. They are so surprised when a bearded stallion approaches them that they refrain from describing their surroundings at all, as though those surroundings needed no description. Furthermore, when this bearded stallion asks that they rule the new nation, they volunteer to do so only as its protectors.
These concepts are all crucial to the young princesses' identity: distant parents, isolation from other ponies, undying sisterhood, freedom from obligation, comfort with their surroundings, and protection out of pity.
The Almighty Tallest
Trouble Shoes notwithstanding, the royal sisters are the tallest ponies in Equestria. By herself, Celestia stands at about twice the height of the average pony. The two sisters are huge.
And yet, they are not so huge. Princess Luna is exceptionally slender. Princess Cadance acquired this characteristic too once she reached maturity. Even Celestia, who has proven wider than Luna, is thinner than the Saddle Arabian delegates—both actual horses.
Finally, both Celestia and Luna have retained a youthful appearance for over one thousand years.
So, in addition to the power to bend the heavens, the royal sisters are almost supernaturally tall, thin, strong, and ageless. Neither Cadance nor Twilight Sparkle can boast to be more than two of these. More on this later.
Magical Myst-ery Cure
When she is not descending in a swarm of bats, when she wishes to speak in privacy, Princess Luna appears in dreams.
That said, it is now time to consider one of the most confusing scenes in the entire series. After completing Star Swirl's spell in Magical Mystery Cure, Twilight Sparkle finds herself in a strange place wherein the only other pony is Princess Celestia. Some have called this place, where strange balls of light shine overhead, a glimpse of outer space. Others believe it is the interior of Twilight's mind.
Whatever this place is, I only recently noticed what may actually be the most important part of the scene: the mist. While there certainly are starry orbs in the background, the rest of the space is filled with a mysterious, bluish mist, and this is what gives the scene its dreamlike quality, even before Twilight's memories come under review.
This is what seals the deal for me: even though the two sisters can draw upon the moon and sun, they still choose to appear in dreams and mist when the matter is personal.
What are the Two Sisters?
No, they are not goddesses. They never were.
Princess Celestia and Princess Luna are, in a sense, Equestria's sylphs—fairies.
The sisters' journal reveals their early life in solitude, where they had nary a care but each other's continuous happiness. It sounds as though they spent their days prior to their coronation frolicking in the wilderness like the nymphs of Greek mythology. Star Swirl discovered these mysterious maidens, and they happened to be just what Equestria needed for leaders. Were they not so touched by his plea, the two sisters might not have come to know pony civilization at all.
Celestia and Luna are tall, ageless, and miraculously fit because they are, in their own right, creatures of myth in a fantasy world. Recall The Wizard of Oz and its depiction of a fairy-like "good witch" who presides over a nation of "little folk." The two sisters fill a role that is all but the same.
Finally, the dreams and the mist are simply common motifs from the romantic era of ballet. Giselle, La Sylphide, and even Swan Lake portray airy, ethereal figures of supernatural origin and grace appearing out of mist before the leads. (Even Pokémon pays tribute to this trope with the Fairy-type move Misty Terrain. Does this scene look familiar to you?) Historically, fairies always appear onstage after the protagonist dozes off or after the mist rolls in. Celestia and Luna seem to share this habit.
Now, bear in mind that Equestria is a fantasy world in itself. There is actually not much supernatural about Celestia and Luna because, in Equestria, magic is natural. Therefore, I dare not to say that the two sisters are simply fairies; rather, I mean to say that they are Equestria's version of a specific facet of age-old lore.
In the same way that Discord is the "spirit of chaos," even though he possesses virtually no spiritual qualities, Celestia and Luna are concurrently spirits and not-spirits. They are based on the myth of fairies, and yet the franchise's Alicorns are distinct creatures in their own right. There are few other words for it.
The two sisters share the attributes of fairies without being fairies because Alicorns are, after all, still ponies.
Thank you and good day. (sun)