Once in a Rainbow Moon

by Pineta

Rating: everyone
Tags: My Little Pony: Friendship is Magic, Flurry Heart, Rainbow Dash, Twilight Sparkle, Princess Celestia, Slice of Life, Princess Luna
Length: 2304

One night in the crystal empire, Twilight Sparkle and Rainbow Dash decide to engineer a blue moon.

Can you make a blue moon? Luna is sceptical, but Twilight Sparkle and Rainbow Dash decide to have a go.

Published 2018-08-02 19:40:01 UTC
Last updated 2018-08-02 19:40:01 UTC

Lunar chromo-engineering

“This one’s the sun… and this is the moo—n…” Twilight took time to clearly pronounce the names of the astronomical bodies while waving the coloured glass elements of a mobile hanging above the crystal crib where her niece lay.

“Moo moo moo—n!” echoed little Flurry Heart, waving her tiny hooves in the air and tracking the crystal ornaments with her large eyes.

“Princess Celestia raises the sun, and Princess Luna raises the moon. The moon moves around the Earth and the Earth moves around the sun.” She set the mobile spinning using her magic to illustrate this dynamic. “The moon doesn’t emit light itself, but it reflects the light from the sun. It goes through a monthly cycle of phases…”

Flurry Heart giggled. Behind Twilight, Princesses Celestia, Luna and Cadance, together with Shining Armour and Rainbow Dash laughed at Twilight’s determination to educate the young filly about astronomy.

“Y’know,” said Rainbow Dash, “she might be just a little too young to take in all those details.”

Twilight turned to face her friend and defend her teaching method. “Just because she is too young to understand doesn’t mean we shouldn’t explain it properly. It’s not right to patronise foals with silly baby talk.”

“Coo-bu-yugle-caga,” said Flurry Heart.

“She’s excited to see so many visitors,” said Cadance. “Aren’t you Flurry?” She pushed her head down and rubbed her nose against the little foal.

“She has A.K. Yearling to thank for that,” said Celestia. “We’ve only come to the Crystal Empire for the big party tomorrow to launch her new book, but we couldn’t miss the chance to see how our little alicorn was doing.”

Twilight turned back to the little alicorn. “What do you know about the moon Flurry?”

“Be-de-bloo-moo,” said Flurry.

“She says the moon is blue,” said Shining Armour.

“You haven’t taught her very well,” teased Rainbow. “The moon’s not blue.”

Twilight chose to interpret this differently. “Good question Flurry! She wants to know what a blue moon is.”

“Right…” said Rainbow Dash. “Err—what is a blue moon?”

“A blue moon is the second full moon in a calendar month,” said Twilight, quoting a memorised definition. “It occurs every two or three years as the year has eleven more days than the twelve lunar cycles.”

Flurry chuckled. Luna snorted.

“I read that in the Night Sky Tonight column in the Manehattan Times,” continued Twilight.

“These newspaper do print a load of rubbish,” said Luna.

“It said that it was defined in the Baltimare Farmers’ Almanac.”

Luna was not impressed. “These almanac writers just make up these things to make themselves feel important. And I’m the one who gets all the stupid letters from ponies who read too many books telling me that I should have made the moon blue ‘cause that’s what it says in the almanac. What garbage! Why should the second full moon in a month be any bluer than the first?”

“Bloo moo,” cooed Flurry Heart.

“Can you get an actual blue moon?” asked Twilight, her indignation at Luna’s attack on book knowledge displaced by a genuine curiosity.

Luna sighed. “My duty is to raise the moon. Its colour is not something that I control. Perhaps the atmosphere can filter moonlight in a way that makes it blue. Ask a weather pony.”

Everypony looked at Rainbow Dash, but it was Twilight who answered, pondering her own question.

“Air will scatter more blue light—that’s why the sky is blue—and when the sun—or moon—is on the horizon it looks red as the blue light is scattered, but you couldn't get a blue moon that way.”

“We do get blue moons here in the crystal empire,” said Cadance. Everypony turned to face her. “At least, it sometimes looks that way to me, when I look up on winter’s nights when the sky is full of snow.”

Twilight thought about this. “I read that dust particles in the atmosphere are sometimes the right size to scatter light in other ways. Maybe tiny snowflakes can do the same thing.”

“A real blue moon.” Rainbow Dash looked at Twilight with a wide grin. “Are you thinking what I’m thinking?”

“Time for an experiment?” Twilight grinned back at Rainbow. “Like those rainbow tests we did?” They exchanged a hoof bump. Twilight turned back to the lunar guardian. “Luna would you mind if we stage a snowstorm to turn your moon blue?”

“You can try,” said Luna with a yawn.

Twilight and Rainbow cantered through a door leading on to a balcony with a view of the city and the night sky. It was a clear cold night with many stars visible as well as the full moon. The moonlight reflected off the crystal city towers and streets.

“What type of snow do we need?” asked Rainbow.

“I—I’m not sure,” Twilight said, “we might have to experiment a bit to see what works. They’ll have to be much smaller ice crystals than usual. We know snowflakes are white so they must scatter all wavelengths of light.”

“Okay.” Rainbow Dash shot off the balcony into the night sky, searching for suitable clouds.

Cadance walked out on the balcony levitating Flurry Heart close to her, now wrapped in a little blanket, followed by Luna and Shining Armour. They looked up at the bright white moon. A few minutes later Rainbow Dash landed on the balcony to join them.

“I started a snow squall, altitude three kilometres, outside city limits. It’s falling right in front of the moon.” She looked up at the sky. “But it’s not blue yet.”

They watched the moon shining behind a swirling flurry of snow.

“The flakes are too big Rainbow,” said Twilight. “They’re reflecting all light. It’s making the moonlight a bit dimmer, but still white.”

“Ha—that was just a first try. I got this.” Rainbow shot off vertically to try again.

“You might not find this so easy,” said Luna. “There is a reason why the expression ‘once in a blue moon’ means something very rare.”

Twilight was not listening but had taken out a notebook and quill and was scribbling equations and diagrams on a page. Rainbow returned and looked back at the white moon with a disappointed pout.

“Geez, why isn’t this working?”

“The size must be critical,” said Twilight. “Big particles scatter all colours, so the result just looks white. Small particles scatter more blue light and would make the moon look red. Maybe if the ice crystals are about the same size as the wavelength of the light—about a micron—they would scatter more red and give a blue moon. I don't know what you need to do to make them that small. Try thinning out an altocumulus cloud at high altitude then move it down in front of the moon.”

Rainbow Dash gritted her teeth and took off again, returning some minutes later with tiny ice crystals covering the tips of her wings. “It’s cold up there. What the hay!” She stared at the moon and pointed a hoof. The satellite had now turned a reddish orange colour. “You said it would be blue not red!”

“Look at the red moon Flurry,” said Cadance. Flurry Heart clapped her hooves, not sharing Rainbow and Twilight's frustration at the wrong colour.

“You must have made the crystals too small,” said Twilight.

“Okay, I’ll give the vapour more time to condense before thrusting it into the cold. That will make them bigger.”

She took off in an even more frustrated fashion, dropping a few blue wing feathers on the balcony. The moon continued to redden.

“That doesn’t look quite right,” said Luna, with a frown. “It’s very red. There shouldn’t be that much scattering from a bit of water vapour...”

Rainbow Dash landed again and looked back to the sky. “You have got to be kidding me!” The moon was still bright red.

“Let’s do this systematically,” said Twilight. She picked up her notebook and drew a table on a blank page. “Start with ordinary snowflakes and then make them progressively smaller step-by-step. But first we need a control. Let’s go back to where we started—can you clear the sky please?”

Rainbow Dash shot off to kick away the clouds, clearing the sky in ten seconds flat, only to return to see an open-mouthed Twilight staring at a still-red moon.

“What is going on? I don’t understand. It should be back to white now there's nothing to scatter the light.”

“Hmmm…” Luna scrutinised the lunar face, bathed in light as red as the sunset, set against the star-filled blackness of outer space. “I’ve seen this before…” She lit up her horn and magically nudged the moon to one side. The red disk juddered in the sky. She then pushed it suddenly to the other way. For a moment it appeared as a white crescent curving around the edge of a red disk, then the red slid into place. Luna smiled now she understood what was going on.

“Where is Celestia hiding?”

She galloped back into the room and leapt over a crystal chaise longue. Everypony turned to watch with no idea what was happened. As Luna landed there was shriek from behind the furniture, followed by a burst of laughter.

“Gotcha sister!” Luna pinning her elder sister to the floor with her hooves. Celestia was laughing out loud. “What do you think you’re doing hiding here disrupting Twilight’s experiment? Is this the way to treat your student?”

“It was too tempting,” said Celestia, recovering from a fit of giggles. “And you were having such fun trying to figure it out.”

“What’s going on?” Twilight, Rainbow Dash and Cadance walked indoors, followed by Shining Armour who was carrying Flurry Heart. They stared at the two pony sisters on the floor without comprehension.

“She’s been making an eclipse!” accused Luna.

“Oh!” Twilight smiled now she understood.

“What’s that?” asked Rainbow.

“A Lunar eclipse,” said Twilight. “It’s when the Earth is directly between the sun and the moon, so the shadow falls on the full moon. It is only lit by the light which curves around the Earth through the atmosphere, which is as red as the sunset.”

“It looks quite spectacular when you see it from the moon,” said Lunar. “A red ring around the silhouette of the Earth.”

“We had no chance of making a blue moon, when it is lit only by red light,” said Twilight.

“But,” said Celestia, “there is another effect. Let me up and I will show you.”

Luna released her sister. They all went back out on the balcony. Celestia’s horn was alight with magic as she tweaked the position of the sun a hundred and fifty million kilometres away on the other side of the planet.

“Look at the rim of the moon.”

“It’s blue,” cried Twilight. Sure enough the side of the moon was shaded with a spectrum of blue and turquoise bands, while the centre remained red. “But how?”

“The Earth’s atmosphere is complex and multi-layered,” said Celestia. “You were right to say the moon is red due to the light refracted through the stratosphere. But above this, there is the thin ozone layer. As light shines through this, it absorbs the red, leaving a fringe of blue light to shine on the moon.”

“That’s amazing.”

As the shadow moved, the colours shifted showing shades of grey, red, and brown. All ponies watched the eclipsed multi-coloured moon in silence. Flurry Heart lay asleep on her father’s back. Shining Armour lent his head against Cadance's mane. The stars shone more clearly in the darkened sky.

Two streets away, on a different balcony, a well-groomed unicorn in a business suit stood drinking a cocktail and also admiring the coloured moon. She was the CEO of Random Horse Publications, visiting from Canterlot. To her side stood a young crystal pony.

“I must say,” she remarked, “while it is a shame that we couldn’t get A.K. Yearling to come to tomorrow’s launch party to promote her new book, it was a splendid idea of yours to arrange a blood moon to mark the occasion. It really fits the title: Daring Do and the Fight for the Crystal Moon. But I’m surprised that you could do that within the budget we gave you.”

“Oh!” replied the junior Crystal Empire district marketing officer. His eyes darted to left and right before he continued speaking. “I mean—yes! For every launch party we always try to think of something really big to get everypony’s attention.”

Author's Note

Further details on the science

See: Blue Moons and Lunar Eclipses

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Cover art sources:

https://www.deviantart.com/90sigma/art/Princess-Celestia-and-Princess-Luna-Conversing-2-354574917
https://www.deviantart.com/dashiesparkle/art/Vector-291-Rainbow-Dash-41-567813407
http://eagle1division.deviantart.com/art/Twilight-Looking-Up-432597744
https://science.nasa.gov/science-news/science-at-nasa/2011/02dec_lunareclipse