She didn't have anywhere particular in mind. She just needed to walk, to be out in the fresh air for a while. She had to relax and calm her nerves before she set out tonight to do what she had to do. Oh how she wished she didn’t have to go.
A simple walk with a friend was the perfect thing to calm her mind. Not being up in her throne room in the royal castle, having everypony call her 'Princess' and 'your majesty'. Well, Wonderwhy usually called her 'Princess' but he said it kindly, as if it was her name and not her title. In the company of such a friend, though, she preferred just Celestia. She hadn't meant to frighten him so by addressing him as 'Bee' but she could understand his misinterpretation of her friendliness.
They walked around Canterlot in silence, save for the gentle clip-clopping of their hooves. They encountered nopony else, as expected at this hour. Finally, long after the pair had lost track of time, they sat together on the grassy bank of one of Canterlot's many streams. The Moon was a bright circle hovering directly above them. The Mare in the Moon cast a beady eye over proceedings. Wonderwhy didn't quite realise just how tangible the dark shapes on the heavenly body's surface would soon become.
Nothing was said for a while more, until Bee remembered that Celestia wanted to tell him something about the Moon. He squinted up at its tranquil form and decided to break the ice. "When I have time to ponder such things, I've often meant to ask you, Princess, about raising the Sun and Moon. For instance, once you've got them above the horizon, is that it as far as you're concerned? Do you just let them trace across the sky, or do you need to constantly think about their movements?"
Celestia blinked but didn’t turn to face him. She carried on gazing off into the distance.
"They require the presence of great magical power to remain in balance. Since they are both under the control of just one alicorn, they can remain perfectly in balance forever. Once I have raised either the Sun or Moon, the influence of the other keeps both in check until it's time for them to swap places. I simply need to be alive and well."
Bee pulled a contemplative face. He was more aware of Celestia's past than most ponies, so he didn’t react much when she continued, "They have been perfectly balanced for one thousand years now, guided by my magic. However," she said with a certain strained undertone in her voice, "it is not meant to be so. One pony's power cannot sustain such balance indefinitely." Bee knew full well what was coming next, yet couldn’t help but twitch his head in surprise when she concluded, "I... Equestria needs my sister back."
"Wasn't Twilight Sparkle reading about your sister the other day? I was walking just near here and I saw her lying on some grass. She was reading a book aloud to herself, and I distinctly heard phrases such as 'two regal sisters' and 'refused to lower the Moon'. Does she know what I do?"
One corner of Celestia's mouth reluctantly turned up. "Probably. You said Brightlight Bluesky was stubborn, but Twilight's not one to give in easily either. She even wrote me a letter asking what was to be done." Bee frowned. "Given that my sister returns tomorrow."
"Whoa, what? Tomorrow?" Suddenly he smacked his forehead with his hoof. "Ugh, I was right after all!" Now it was Celestia's turn to look puzzled. "Ah, you see, I was going over some numbers I came across in various ancient prophecy books, and out of curiosity I used them to calculate when Princess Luna should return, if the 'thousand years' part of the legends are true, and my answer was in fact tomorrow." He sighed. "I should have said something, but I assumed I was wrong." He smiled. "And of course you obviously knew it would be tomorrow, so I needn't have informed you anyway." He was jolted from his rambling by a drop of water landing on his hoof, which he quizzically raised in front of his glasses. There was indeed a small wet patch, but the sky was clear of clouds. He shrugged. "So anyway, what did you tell... Twilight?" His voice faded to a mumble when he saw the Princess had closed her eyes. "Celestia?"
The mystery of the water drop was solved by its companion peeking out from between Celestia's eyelashes, before it too plummeted earthwards with an inaudible plop. "My sister was lost to the world when her power became corrupted, and she transformed into..." The Princess momentarily choked up. Bee tried to comfort her by way of nudging his hoof into contact with hers. "I did not banish Luna; instead I rid the world of Night Mare Moon, and it is she, not Luna, who will return tomorrow."
"Sorry," mumbled Bee, "I just called her that because... well, I don't know. I'm sorry." He retrieved a hoofkerchief from one of his orange vest's many pockets and hesitantly offered it. She smiled weakly in reply and it floated across to her. Bee averted his gaze, embarrassed.
"My sister was only able to become so... misguided, shall we say, because our powers over the Sun and Moon grew terribly unbalanced. The price of sharing the burden of my task is that the forces inevitably shift into different patterns." The hoofkerchief magically wafted to the stream and washed itself. Bee watched as it wrung out the water and folded in midair, before drifting slowly up the bank and into his pocket.
"So why did something like this not happen earlier? Why was a thousand years ago the first time your powers got so uncoordinated that they corrupted?"
"Strictly speaking, the imbalance required correcting at least twice a year, sometimes as often as five times in one year. We couldn't do it ourselves, though. We needed a third source of great magic to fix the differences between us."
"A third source?" asked Bee in amazement. "Who could you possibly call on for that? Surely only alicorns possess the power to control the Sun and Moon. Well, you could have assembled thousands of unicorns I suppose, but you said 'a third source' of power, not thousands. Who was it?"
The Princess inhaled sharply. "His name and life have been lost from general knowledge for countless generations." She relaxed again. "I will never forget him though. It was so shocking when he turned against us, we were as close as siblings could be."
Now Bee stiffened. "Wait, siblings?"
Celestia nodded slowly. "I had a brother once." Her eyes suddenly widened as she looked up at the Moon, which was now well on its way down again. "I still do, really, and if I don't get going right now, the rest of Equestria will remember him too."