"Luna!" boomed Celestia, Princess of the Sun Most High. A comically oversized clothespin was clamped to her nose, half to make a sarcastic point and half because her little sister was now classified as a biohazard. "Bathtime. Now."

"You speak in riddles, Sister," said Luna, Princess of the Moon Slightly Less High. As the younger sister, her clothespin was but moderately oversized. She flipped through a page of Air Fresheners Quarterly and gave a casual sort of shrug. "I have never before heard of bathtime, as you call it. Praytell, is it a modern slang? Have the hip young foals of today decided that things are totally bathtime?"

"No games, Luna." Celestia swatted the magazine from her hooves and grabbed Luna magically by the mane. Her filthy, filthy mane; even the constellations were covered in dirt. "Baths existed a thousand years ago, and it's high time you took one. They can smell you all the way from Ponyville."

"Only when they are downwind!" Luna cried, struggling frantically to escape. "You are overreacting, Sister, and more importantly, you are wrong. It is natural for princesses to embody their element. Cadance, as Princess of Love, is deeply in love with her husband. Twilight Sparkle, as Princess of Friendship, has many friends. It is only logical that I, as Princess of the Moon, should not be allowed near water, since the Moon has none."

Celestia cocked an eyebrow at that. "Okay, Luna, and if you don't take a bath in the next ten minutes, I, as Princess of the Sun, will erupt into a solar storm that scorches the earth upon which you stand. Equestria will become as a wasteland, inhospitable to all life, and we'll sit here for however many eons it takes for civilization to return and reinvent the bathtub." She leaned down and hissed in Luna's ear: "Do you want that to happen again?"

"No," Luna huffed. "It took long enough to get back to the wheel. Fine, Sister. You may have it your own way. I shall go to my chambers now and undertake a bath."

"Oh, we're well past that," said Celestia, refusing to let go. "A situation like yours requires far more extreme measures than a simple bath." Unsympathetic to Luna's struggles, Celestia dragged her little sister along the floor. A thick trail of viscous slime followed where Luna dug in her hooves, which sizzled quietly as it ate into the floor. Ponies in hermetically sealed suits, specially trained by the Royal Department of Pollution Containment, followed behind to clean up. Celestia shook her head sadly as one pony collapsed, his filters damaged by the corrosive residues. There was no time to stop for the fallen. Everypony on that team had known the risks.

"Really, Sister?" Luna muttered. "You do not think all this a little too overdramatic? His acting was terrible!"

"Brave soldier that he was," Celestia whispered back, a tear forming in her eye. "We shall not forget his sacrifice."

Down they went to the deepest sub-basements of the castle, where the Royal Department for Treatment of Toxic Wastes kept its office. Guards saluted them as they traversed a set of white-walled corridors, all the way to the Department's deepest and most secretive door. "Username: Princess Celestia," Celestia spoke aloud, placing her hoof on a biometric sensor. "Password: Password. Authorize the operation. It's time."

"Identity verified. Authorization accepted," said a computerized voice. "Final confirmation required: Are you sure?"

Celestia nodded solemnly. "I'm sure."

"Confirmed." The door lit up green, and Luna was admitted inside by a team of Equestria's finest, armed to the teeth with steel wool, buzzsaws, and industrial-grade detergents. They had prepared for this day since the time of their great times eight grandfathers, and after a thousand years of preparation, Operation B.A.T.H.T.I.M.E. was finally ready. The only thing left to do was pray.

For what seemed like hours, Celestia paced the hallway outside the door. She could hear the muffled whir of the buzzsaw through the door. Luna's screams echoed throughout the hall. "It's for the greater good," she told herself, as if that somehow justified what she'd done to her sister. What a monstrous sacrifice she had made. A bargain with demons. Yet, in her hooves lay the fate of every pony in Equestria. What was the measure of Luna's hygiene, compared to thousands of ponies' noses? How could she have stood idly by, allowing Luna to leave her dirty, unwashed socks lying everywhere? She nodded solemnly. "It had to be done."

Eventually the door lit green once more. A sullen-faced doctor stepped out, a clipboard held in his magic. "Are you ready?" he asked her.

"As I'll ever be." Celestia gulped. "What's the diagnosis?"

"We're underappreciated and underpaid," said the doctor grimly. "And your sister called us a roving pack of clean-freak nerds while under anesthesia. None of us actually like her. I fear it's only a matter of time before the uprising begins."

Celestia nodded. "But what of my sister? Is she okay now?"

"We did everything we could..." said the doctor. "And I'm happy to announce that we're awesome. Your sister is completely clean! Not a speck of dirt left on her. Totally spotless, except for her attitude. Nothing to read into there."

"Oh, thank goodness!" Celestia let out a heavy sigh. "That's wonderful news. I'll have the cooks prepare a celebration."

"But!" said the doctor, holding up a hoof. "But..."

Celestia inhaled sharply. "What about her butt, doctor? Tell me, please!"

"It's very curious." He flipped through the clipboard and held it out to her. "It turns out that what we had previously thought was the Princess' cutie mark was actually an enormous black stain."

Celestia's eyes grew gradually wider as she flipped through the clipboard. Pictures had been taken as record of Luna's flank during the bath, the inky-black splotch of her cutie mark fading more and more with successive scrubbings. She stopped short of the very last page. "Tell me doctor," she said. "What does this mean?"

"It means that Princess Luna's cutie mark is not the moon after all," said the doctor grimly. "It is actually..."

"No..." She flipped to the last page. An opaque blue sphere stared back at her from Luna's flank. All breath fled her lungs. "No!"

"Yes!" said the doctor. "Uranus!"

A quiet settled over the hall. Celestia coughed. "No, all right, I'm sorry, doctor, I must have misheard you. This looks nothing like my--"

"Sister!" Luna cheered, practically twirling out of the room. "Have you heard the news? What a grand and amusing mistake! To have thought myself stewardess of the moon this whole time!" She laughed. "That silly old night. Nopony liked it anyway. I am only too proud to accept my true role as guardian of Uranus."

"Luna!" Celestia gasped. "That is extremely private!" She narrowed her eyes at the doctor. "This information is to be classified. Nothing my sister just said leaves this Department. Ever."

"Yes, I agree, Sister!" said Luna, eagerly pushing the doctor back through the door. "The commoners should hear of this only in a grand announcement. No rumormongering shall be necessary."

"No, that's not what I meant at all," said Celestia, flipping again through the clipboard. There again, that turquoise-blue sphere stared back at her. Was that truly Luna's cutie mark? It was hard to say. She could barely remember a time when Luna wasn't covered in grime. "I don't know what those so-called experts have told you this mark represents, but it is most certainly not my, nor is it anypony's--"

"But mine!" finished Luna, sparkles in her eyes. "Ah, I cannot wait, Sister! Perhaps it is merely the detergent fumes speaking, but I feel as though this is a fresh start for me! No longer shall I be custodian of the pale, boring moon. No, by this time tomorrow, every pony in Equestria shall have heard the proclamation: I, Luna, am now Princess of Uranus!"

Celestia opened to her mouth to object, then closed it again. What was she to do now? She'd never seen her sister so happy and/or clean in a thousand years. After subjecting Luna to such horrible trauma, what kind of monster would she be to steal away her newfound joy? To make Luna associate negative feelings with being clean? Doing something like that would make her no better than Tirek, or Super Tirek, and she wasn't about to be that bad a sibling. Now was the time to be supportive of Luna's new, uh, passion. She could ride this one out while she researched whatever Luna's cutie mark actually was. Nothing was happening now that she hadn't survived before.

"Well, Sister?" said Luna. "Shall I begin preparations for the announcement? May I prepare banners? May I commission new portraits of my true mark? May I order the poets to scribe loving odes to Uranus?"

"Why, uh..." Celestia gritted her teeth. Bury all feelings. Sublimate the pain. "Yes, Luna. Whatever makes you happy."

It was going to be an impossibly long week.

"You don't need to do this, Luna," said Celestia. She flipped anxiously through her copy of Seven Ways to Cope When Your Little Sister is Apparently Some Kind of Mad Fecal Pervert, but none of the advice seemed particularly relevant. "Really, we can have a quiet private party and ease the public into this. There's no need to go all out like this."

"Nonsense, Sister!" said Luna, brandishing her royal announcement scepter like a club. "This is a momentous occasion for us, nay, all Equestria. Our subjects must know to bow in proper reverence to Uranus."

"Which is precisely what I'm worried about," Celestia murmured, casually ducking Luna's swing. "Just make something up. Tell them your cutie mark is a new planet, or something. It looks kind of like it would be one, I guess." She squinted at the blue sphere imprinted on Luna's flank. Something about it still didn't look right. "Kind of."

"Ha!" Luna ha'd. "I see now that the horseshoe is on the other hoof, Sister! It is you who are out of touch." She patted Celestia playfully on the head, using her heavy onyx scepter. "Worry not. This is a momentous occasion." She strode out onto the balcony, leaving a very dazed and bemused Celestia behind.

"Citizens!" Luna announced, to hideous booing. A tomato whizzed past her head. "I am proud to announce to you that I, Princess Luna, am finally clean of dirt! No longer shall I fear the bath as I once have. Bathing shall now occur for regularly, at a minimum guarantee of once a century!"

Begrudging applause from the audience. Some of them still remembered the family they'd been separated from under the quarantine.

"That, however, is not why I have called you all today!" she shouted. "No longer I am Princess of the Night. In my bathing it was revealed to me that my true calling is not, and has never been, related to the moon!"

A hush swept across the crowd. Now that was news. Where could she be going with this one?

"No more shall it be my job to raise and lower the moon. That task shall instead fall to my sister, who has performed it beautifully for the past thousand years before my return anyway." Luna chuckled to herself. "It's quite humorous, in retrospect. I know that many ponies did not appreciate the night time much, and for so long that upset me. But now, after a thousand years, I have finally learned to turn the other cheek. It is time to begin a new chapter in my absurdly long and endless life. I am not a guardian of the night. Princess Luna is, in fact, a guardian of Uranus!"

"Luna." Celestia tapped her on the shoulder. "Please stop talking."

"Yes, Uranus!" Luna went on, increasingly animated. Flecks of spittle flew from her lips as she shouted. "Should any problem, however small, threaten Uranus, it shall be my celestial duty to defend it! Any foreign intruder upon that sovereign territory shall be dealt with swiftly and without mercy! Nay, I shall wield all of my power as a princess to wipe them from existence! Let Uranus be considered from hereon as a national treasure, as my personal symbol! I, your Princess Luna, promise to you all that I shall fight to my utmost to ensure it the respect it deserves. Uranus now, and Uranus forever!" She bowed. "May we all carry on, for the glory of Equestria!"

The crowd remained silent. For a solid minute, even the crickets had nothing to say.

Clop, clop, clop.

In the back of an audience a lone mare had begun applauding. Then another, and another. Applause spread throughout the crowd like a virus, until the whole crowd could do nothing but clop in approval. Luna bowed once more in favor of the uproarious applause. "Thank you all!" she cried. "Thank you all so much!"

"Okay, show's over." Celestia ushering Luna back inside. "Merry April Fools' Day, everypony! I hope your own pranks are half as good!"

"Um, Princess?" a guard whispered to her. "It's January."

"What a prank!" Celestia cried loudly. "We've been had! It's actually January. Good one, Guard!" She chuckled nervously and stared out across the crowd. "No? Nobody convinced?" She sighed. "Fine. You got me again." Her horn lit up and the million-ton sun in the sky changed trajectory. Most of the snow evaporated on the spot, and ponies suddenly found their winter coats stifling. "April Fools! It's actually April! Happy pranking, everypony!"

"Really, Sister," said Luna, cocking an eyebrow as she stepped back inside. "I should hardly think it was necessary to cut my speech off at the end. Just as they were applauding for me, even!"

"Luna," Celestia explained, as gently as she could. "They weren't cheering for you, they were cheering at you."

"Is this a modern grammar lesson?" Luna frowned. "Fine. Just as they were applauding at me. Now explain. What has gotten you so on edge? I am not blind, Sister. I can see that you have had some reservations over my celebration of this revelation. Now it is time for us to have a conversation. Why did you interrupt the most important day of my life with the punchline to your epic prank? Explain yourself."

"Oh no..." Celestia rubbed her temples. "I didn't think we'd need to have this talk. Ever. I'm not prepared for this, and neither are you, to be frank, but it has to be said before this gets even further out of hoof. I don't know what you think Uranus is, but a pony's anus is a very special, very private--"

"Ew!" Luna retched. "Sister, control thy tongue! My celestial symbol is Uranus, not your anus! Gross! How conceited and depraved could one be to think that? It hardly looks anything like your anus! It is very clearly a planet, and the seventh planet to revolve around the sun at that! I should have expected you, of all ponies to know this." She harrumphed and trotted off. "We shall speak no more of this. Be happy for me that I now service Uranus, instead of that boring old moon. Good day to you, Sister!"

Celestia's mouth fell open, and it stayed that way for a while until a bug flew in and she had to cough it up. She was shocked! Just shocked. Never before had Luna spoken to her like that. More importantly, what was it she had said? "Seventh planet..." she repeated. "Revolves around the..." Her eyes widened. "Oh."

"Luna?" Celestia cracked the door to Luna's chambers, a bulging saddlebag at her side. "Are you in here?" The bed was covered in pamphlets from the Royal Department of Asstronomy, covering Uranus. On the nightstand, she noted a copy Seven Ways to Cope When Your Older Sister is Apparently Some Kind of Mad Fecal Pervert, and a vein twitched in her temple.

"Sister!" said Luna, cheerfully bursting from the stack of pamphlets. "It's good that you're here. I've become so popular ever since announcing myself as Princess of Uranus! All our subjects laugh and cheer with mirth when they see me now, instead of throwing up from my body odor!"

"Oh..." Celestia forced a smile. "That's good to hear, I guess? But I, um, wanted to talk to you about that, actually."

"Of course!" said Luna. "I have just been doing some light reading on the subject myself. For instance, did you know that Uranus is full of gas? Or that fifty copies of our moon could fit within Uranus? Or that--"

"All right, first of all," said Celestia, "please stop. Secondly, we really do need to discuss Uranus."

"My anus?" Luna pursed her lips. "Or Uranus?"

"Uranus."

"My anus?"

"No," said Celestia. "We're not doing this." She conjured an enormously thick astronomy textbook up from her saddlebag and let it fall to the floor. It crashed through the tile, and straight through several more floors down below. The two of them stared briefly at the hole. "When someone gets that," she said, "you're going to read it until you understand basic astronomy."

"Pfft," Luna scoffed. "As if I needed the update. Must I remind you, Sister, that--"

"The sun and the moon revolve around the Earth," Celestia finished. "Every foal learns this in elementary school now. There's no such thing as Uranus. It's fictional."

"No!" Tears welled in Luna's eyes. She scrambled to pick up her pamphlets. "You are lying! The Royal Department of... Wait." She read the title again. "The Royal Department of Asstronomy!" she cried. "Not even their pornography is peer-reviewed! How could I have been so blind? There weren't even any citations!"

"It's all been an elaborate prank," Celestia agreed solemnly. She sat down on the bed and stroked Luna's mane gently. "Apparently every pony on the castle's staff was in on it. Mostly because it seemed funny at the time."

"B-but the mark!" Luna choked out. "What about my cutie mark?"

"Even the mark." Celestia magicked a water bottle from her saddlebag and squirted it on Luna's cutie mark. The blue food dye ran off instantly. "It's not Uranus. It's a full moon. "

"Not the boring old moon!" Luna wailed. She buried her face in her pillow, muffling her sobs.

"Yes, the moon," said Celestia, patting Luna softly on the back. "But there's nothing wrong with that, Luna. You said it yourself: a princess must embody their element, and you embody the moon perfectly. You're mostly active when the Sun is down, you frequently avoid surface water, and you're always the brightest thing in the night sky."

Luna choked out another sob. She looked up at Celestia, her cheeks wet with tears. "You mean it, Sister?"

"I mean it," Celestia nodded. "No matter what other ponies say, or think, or smell, you're always going to be my little sister. It doesn't matter if our subjects like you better for Uranus. They're not going to see Uranus when they stare up at the sky, they're going to see the moon, and it's always going to be beautiful." She wiped Luna's tears and gave her a quick kiss on the cheek. "You get that?"

"I got it, Sister," Luna nods, still sniffling. She smiles. "Thank you."

The clock rings as it strikes six. "And just in time, too," said Celestia, smiling back. "Why don't you go out there and give all our subjects an eyeful?"

"Of course!" Luna leapt to her hooves, scrambling to the door. "Look out, Equestria! Tonight, your princess shall moon you, and it will be the biggest and most beautiful moon in all history!"

Her sister consoled, Celestia walked to the window and stared out at the starry night sky. A million billion celestial bodies were shining down on Equestria that night, and every night. And if she focused on one little blueish-looking spark in particular, she could have almost sworn that... Well. She shook her head. No, just a trick of the eye. It couldn't be, and it didn't matter now anyway.

"You moon 'em, Luna," she cheered silently. "You moon 'em good."