(Georg)
"Actually... I must confess something, Twilight." Celestia lowered her voice and checked to make sure Luna had finished going down the hallway to her room. "You are quite incorrect. I do have a shadow."
"But you don't!" insisted Twilight. "It's perfectly plain. Even now." She waved a hoof under her fellow alicorn. "See!"
"My shadow is not here, Twilight. It's rather... complicated." Celestia took a short breath. "It's Luna."
(horizon)
Twilight paused for a moment, then laughed uneasily. "Okay, that was a good joke, Princess," she said. "But it's a little too unbelievable. I mean, Luna has a shadow." Twilight pointed discreetly down the hall, where the other royal sister was walking from her room toward the royal kitchens — a dark blotch on the ground underneath her, exactly opposite her body from the hallway lights.
Celestia nodded neutrally. "Twilight, you've got an excellent grasp of my sense of humor. But I can assure you: the fact that there's no darkness underneath me is no prank, nor a trick of my magic."
Twilight stared at Luna's receding form for several seconds. Then swallowed. "… You're serious."
"It's a funny thing," Celestia said gently, "how alicorn shadows work. Just like with ponies, they take the shape of a mere dark patch underneath our bodies where we block out the light — until we are in the proximity of others who bring out our deepest emotions. Then they manifest physically as our greatest hopes and dreams. Or as our greatest fears. Or, sometimes… just as the way we see ourselves, in our heart of hearts."
Twilight's wings fluffed out a bit. She rocked side to side on her hooves, trying not to let her nervousness show. "What are you saying, Princess?"
"My faithful student," Celestia said — pointedly using a title she hadn't called Twilight since her coronation months ago — "now that we're standing here together, do you know where your shadow is?"
Twilight looked down. Her blood froze in her veins.
Underneath her, the marble floor was a pristine, undarkened white.
Twilight's mouth opened and closed several times. She looked at Celestia, and at the bright floor underneath her. Then down at herself and her own floor. Back and forth a few times, as if she could make the effect go away by looking again.
"Oh," she finally said, voice weak.
Luna paused in her trek back to her room — plate of pie hovering in midair as she chewed — and glanced over at the two alicorns quizzically. She opened her mouth to say something, paused, shook her head, and walked off again.
Twilight's eyes strayed down to Luna's shadow as the dark alicorn left.
"Oh," Twilight said. "Oooooooohhhhhh."
(Georg)
Luna merely rolled her eyes and kept walking. After all, Celestia was the one who had suggested she take up a hobby of a sort that could not go wrong under any circumstances. And her new collection of pony shadows would not sort themselves.