Celestia Always Wins.
It was a beautiful end of the day for all of Equestria. All the ponies, big and small, prepared to begin heading home to relax until the night was upon them, and then until the unstoppable eventuality of sleep was upon them. For two immortal alicorn rulers, however, such a night was never uneventful.
"You can do it, sister! Thy formidable adversary has never beaten you once, and it will not beat you tonight!" The dark alicorn, Luna, shouted out to Celestia. Luna never grew tired of this spectacle, one of the solar mare's countenance tucked into a grimace and squinting at the sun everypony believed only took a whim from her sister to lower.
The truth was far from that. It had started a few hundred years past a thousand years ago when the unicorns were still using magic to lower the sun. The sun, by Celestia's own words, was large and stubborn, which was why when you stared at it, it stared back and burned your eyes. So one morning, after being called the most stubborn pony Luna knew of, Celestia decided to initiate a staring contest with the sun after looking up.
After six hours Luna had grown worried when her big sister hadn't come back inside the house, she had found Celestia with this exact face staring up at the sun as it gently lowered itself below the horizon. It was thence that Celestia had gained her cutie mark. A joyous occasion that they had celebrated all night until they had fallen asleep exhausted until they found out the sun would automatically rise each day.
From then on, Celestia would spend each day staring the sun into submission, and they had made a game of it, usually with Luna guessing how long it would take for Celestia to win. Every day that number had gotten shorter and shorter, and now the sun would submit in nearly an hour's time.
Back to the present, Luna was cheering her sister on, tapping her shoe-covered hooves together. It had only just begun and the two were even and even. The sun with its single eye, and Celestia with her two burning orbs. Each had their own advantages, and each had their weaknesses. What those were Luna could not tell, but she was always excited for this regardless.
Celestia squinted one eye as the sun stayed still, not cowed in the slightest.
"A brave front sister, put more stubbornness into it! The dastardly sun will not best someone like you!"
Celestia's right eye twitched, but never fully closed at Luna's words. As much as she loved having her sister back, she was still not used to the cheering that had stopped a thousand years prior. the solar monarch had been doing this in silence for three hundred sixty-five thousand and some such days. She had occasionally failed Luna's challenge during earlier times, but tonight was different. It was time to finally show Luna that she was the better light in the daytime as the lunar mare had chosen an hour, the lowest she had ever gone, for tonight.
Celestia couldn't disappoint her little sister, and if there was one thing that Celestia was even more stubborn at than looking at the sun, it was never disappointing her sister again. Already she had failed Luna spectacularly, now she would prove herself to Luna, prove herself to herself.
If the sun could sweat it would, five minutes had passed and Celestia was nowhere near her limit. It knew that its descent was an eventual certainty, every night it would resist as long as it could, but this darn mare just wouldn't give up! Didn't she know that you were supposed to go blind upon seeing its radiance!? Didn't she know that its Magnificence was too large for mortal eyes to gaze upon? No, she had to go and become immortal just to spite it, just to prove her own superiority over it.
"Haha! Cretin, you are no match for mine sister!" Luna swore as she lifted her shoe-covered hoof to point at the sun, taking care not to look directly at it. She was not as stubborn as her sister, nor so mortally challenged. Her sister didn't even use any eyedrops or water! It defied all logic and expectations, and that's what she found most invigorating about all this. The sheer craziness was enough to make this fun.
The sun dipped slightly before wobbling and moving back up. It was so imperceptible that only the two immortal monarchs of night and day noticed it. A smirk found itself upon Celestia's face. Only twenty minutes had gone by and already the sun was starting to lose its ground. As she widened her eyes a bit more, she craned her neck farther out, as much as the two inches would help, to the sun.
Luna had gone utterly quiet as the tension between stellar object and mare could be cut with a knife. Celestia was forced to relax her eyes as the sun brought forth a burst of willpower from somewhere, keeping it stabilized in place. She counted that she only had twenty-five minutes remaining until the hour mark, and so she kicked it into overdrive. A look she had promised to never use on anypony again as long as she lived.
The look of utter horror that graced Luna's face and caused her to avert her own eyes and cover them with her hoof. If a maid had happened to walk by, they would have fainted on the spot, for Celestia was giving the sun a deep stare of disappointment. She had once used this look on Luna and had to apologize with her entire allowance of sweets.
The sun wobbled, having never seen this look from the stubborn mare, it felt an impossibly icy chill in its core of hydrogen, and gradually, slowly and gently, it began to lower. Soon, it could be seen no more.
And in forty-five minutes no less.