When it was just the two of them, silly titles like "Princess" were dropped. She was Auntie Celly, after all, and they both were princesses. And when it was just the two of them, Cadence could ask her Auntie Celly anything at all.
Except there was one question that Celestia would never answer properly. It wasn't a personal question or anything embarrassing or anything like that, but whenever Cadence would ask Celestia why the Sun didn't move on its own like it does in the legends of the Godlings, Celestia would shake her head and say "I'll tell you when you're older."
Well, Cadence was older now. She was engaged to Shining Armor and they were to be married next week. She figured it was finally time to learn why the Sun didn't move on its own. She knocked on the door to her aunt's room in her special way that meant "Let me in, Auntie Celly".
The door opened slowly, and in the pale moonlight Cadence could see Celestia laying on the floor, reading the latest letter from Ponyville. "Hi, Cadence." She said softly.
"Hi Aunt Celestia." Cadence said, closing the door behind her. "I have a question, and I know you can answer it for me."
Celestia closed her eyes. "I was wondering when you'd ask me this again."
Cadence was undaunted, but drew close to her aunt. "Why must we raise the sun? It moved on its own for the Godlings. The moon, too. And all of nature. So why must we ponies do by hoof what was given to them?"
Celestia sighed. "It's time you learned the truth about our world, Cadence. I had hoped to only tell this once, to you and to Twilight at the same time, but I can see that you have seen enough of life to understand what sacrifice really means. Twilight, as much as I love her, is still not ready for this truth."
She raised her head and stared into the flames, the letter placed gently to the side. "To answer simply, we must raise the sun and moon, we must change the seasons, we must care for this world because it has been brought back from the dead."
Cadence tilted her head in confusion. "I'm... not sure I understand. This world was dead?"
Celestia nodded. "We are not the original stewards of Oath, Cadence. This was once a planet for the Godlings; it was a part of The Infinite Experiment."
Cadence swallowed dryly. She had been taught of the Godlings and of the Infinite Experiment. "But this wouldn't have been their Final Home, then, if they left here."
"They didn't leave. They all died." Celestia said sadly. "The first great secret of Oath is that this planet died with the Godlings that had been here."
"But what killed them?" Cadence asked. "I know that the Godlings were warlike and prone to self-damage as a species, but not to the point of self-destruction."
"The second great secret of Oath, Cadence, is that magic draws from the life around it." Celestia said. "We use magic, but not for free. Part of the price we pay for magic as we have it is our stewardship of Oath. We take care of this world, and we ensure that more life lives than is lost by magic. The Godlings found magic and abused it, until even the sun that Oath orbited grew dark and dim from the misuse of magic. The planet had become a wasteland and a prison."
"But you said this planet had died. Can magic do that, draw life from the planet until even the planet dies?"
Celestia shrugged. "I only know the story as it was taught to me. It's entirely possible that it's all a load of hooey, but I believe that there is truth to this. After all, the First Godling was the one who told this story to Luna and I. He warned us not to overuse magic, and warned us of the most dangerous being in Tartaros."
Cadence shuddered. The Ancient Demon, the Ravager of the Ages, the Original Defiler. Many titles and false names were given to it, but only a select few - a number that decreased whenever a Timeless One died - knew its original name. "You still haven't told me why you must raise the sun each day."
Celestia did not smile. "Because it is not a star. It orbits Oath based on the diligence of ponies in caring for Oath. Oath orbits a dark sun, a star that has been dead for many millions of years. I do not want to see Oath become Athas again."