Evening
Raven Inkwell glanced at the banquet room's dinner table, upon which Luna had laid out a detailed map of the night sky. A few stars were circled in red ink. Near them, pencil marks showed proposed new locations. "I see you're still working on your...project."
Luna nodded. "I am."
"But be sensible. There's no reason to believe anycreature can survive being banished to the sun. Especially after all these years. It's simply not possible!"
Luna looked up from her work. She said calmly, "If anypony can survive there, my sister can."
"Even if we could get her back somehow...which we probably can't, at least not alive...have you thought about the consequences?"
Luna nodded. "I've thought a LOT about consequences, for many years now. I've thought of how my sister must be suffering, all alone. I've thought of how she must miss our world. I've thought of how, surrounded and tormented by the sun's fires, she must be yearning for a simple drink of water."
"You're impossible! Celestia was banished to the sun for a REASON."
"Yes, she was. But it wasn't all her fault. Deep inside, she's a good pony. I know she is. I failed her."
"It's not your fault she went mad!"
"How can you know that?"
"How can you know it IS your fault?"
Luna took a deep breath. "It's what my heart tells me. If I'd been more understanding of the burdens placed upon her...the responsibilities, the expectations...our lives could have gone differently. I'm sure of it."
"You're just as mad as she is!"
Luna asked, "Do you really believe that?"
Raven lowered her gaze. "I'm sorry, your highness. It's merely...I fear for what overwork might do to you, too. Even if your powers are less than your sister's were, you're the only remaining alicorn who can move heavenly objects at all. I know it must be a great burden, to give us both day AND night, and to govern..."
Luna waved a wingtip. "Worry not, Raven Inkwell. I have an advantage Celestia never had. I have you, and your tireless assistance. Ponies like you are my best protection. Unlike Celestia, I know I don't have to do everything myself."
"You say that." Raven grimaced. "But on top of all your other labors, every evening you move each and every star so painstakingly, into an exact pattern that's just SLIGHTLY different from any arrangement you've ever tried before. It doesn't matter, Luna! As if a precise hundredth of a degree in the placement of a star will make any difference!"
Luna retorted, "For my sister, it might mean everything."
"You can't know that!"
"But for as long as I care about my sister, I must try."
Dawn
In the darkness, a final grain of sand fell through an hourglass' neck. Beside it on the nightstand, two clocks ticked. Inside one of the clocks, a bell chimed.
Luna's eyes opened. Many years of practice, now, had made her an early riser. She climbed out of her large, rumpled bed, ignoring the tangled heap of sheets and blankets that had collected on the bed's other side. She would take care of that later.
Luna tapped each of the two clocks with one forehoof, to delay further chimes and bells for another twenty-four hours. She turned the hourglass over.
A soft cyan light wreathed Luna's horn. Her telekinetic magic lifted the latch of her bedroom's Prench doors. She opened them, and walked out onto her balcony.
The night was quiet, and calm, and dark, and cold. Luna closed the Prench doors behind her.
Luna gazed at a dim, distant star near the horizon. Her hornlight brightened. The star moved slowly through the sky, until it touched another.
Luna turned her attention to another star, which slid across the sky to touch the first two.
Luna patiently moved star after star, one after another, gathering them together. As each star moved through the sky, a tiny matching star crept through her mane.
Any one star's light alone was cold and dim. But when Luna gathered enough stars closely enough together, they seemed to encourage each other. As the collection grew, each star in the grouping brightened.
Beneath the balcony, a few ponies hurried to their early morning appointments. A baker in her whites, some other kind of worker wearing coveralls. A lamplighter used a long pole with a cup on the end, to extinguish each and every streetlamp.
In the sky and in Luna's mane, darkness waned. The heavens changed color from midnight black, towards a dim, purplish tone, then a lighter purple. In and around the brightening clump of stars, different colors appeared. The glowing clump turned reddish orange, then yellow. Colors seemed to spread and reflect within the part of the sky nearest the starclump, tinting that entire region of the heavens.
As Luna gathered the last few brightest stars still visible, and added them to the clump, the sky turned pale blue. Day had begun. Luna's painstakingly assembled light source was so bright, a pony couldn't bear to look directly at it for even a second.
Something in Luna's mane, too, glowed so brightly that nopony would be able to comfortably look directly at her.
Luna sighed. If this was the cost of keeping Equestria alive and running, so be it. Years ago, she had failed to save her sister. So what if, each day, nopony would or could look Luna in the eye? Being a pony marked apart, forced to atone every morning and day and evening for her mistakes, was probably no better and no worse than Luna deserved.
Luna looked out at the day, and watched ponies walking and trotting through her capital's streets. No matter how terrible a mistake Luna had made, at least she hadn't ruined everything. The ponies living and thriving below proved it.
Luna thought about her next task, and she smiled for a moment. She turned around, reopened the Prench doors, and walked back inside.
Lit mostly by Luna's mane, the room was flooded and drenched with daylight. In the pile of sheets and blankets that still covered one side of the bed, something stirred and rustled.
One edge of the heap rose a few hoofwidths, and a purple unicorn face peeped out. The unicorn blinked in the room's glare. She stuck her head out all the way, then her neck. When most of her body finally emerged, she squinted directly at Luna's face and mane.
After less than a second, the unicorn's eyes closed tightly. She reached over to the nightstand on her side of the bed, and fumbled around until her hoof found a welder's mask, which she put on her head.
Luna didn't see the unicorn's smile beneath the mask, but she could guess exactly how it looked, from having seen so many of the unicorn's smiles before.
The unicorn said, "Good morning."
Luna gazed out past the balcony, rechecking the sky for any little mistakes. "Yes," she said. "It is."
The unicorn crawled the rest of the way out from under the heaped covers, and out of bed. She lifted one forehoof to gently touch the alicorn's cheek. "I know it hurts," she said. "But I know every morning and every evening, you do the very best you can. And someday, when the stars are finally right, we'll be able to try again. We'll be able to bring her back. I know we will."
Author's Note
Bad Horse's "Experience" (link) helped to inspire this story.
(There, now I've made it seem more derivative and half-squelched my chances to win. 
But seriously, I like to acknowledge good stuff.)
