Any solarium is designed to capture as much daylight as possible. One designed by and for Celestia herself, at the top of the tallest tower of Castle Canterlot, could blind any unwary visitors who found themselves within the carefully crafted lensing walls between dawn and dusk.
Luna trotted in at high noon like a drop of ink running down white gold, with about as much care for the ambient light. "Sister," she said.
Celestia, who to an outsider seemed to sit in the middle of the room with her eyes closed and halfway to nodding off, gave no reaction beyond saying "Is something the matter, Luna?"
The all-encompassing glare might not have bothered Luna, but it still took her a moment to adjust enough to see Celestia clearly. She took a step back, ears drooping. "I can come back later if I am disturbing your meditation."
That got a hint of a grin from Celestia. "I have developed some ability to multitask over the centuries, Sister. What is it?'
Luna cleared her throat and straightened up. "There was a worrisome amount of energy buildup in that orbiting human cathedral."
Silence hung in the air for a moment, roasting in the sunlight. Finally, Celestia said, "'Was'?"
"Oh, I already smacked it with the moon," Luna said with a too-casual shrug of her wings. "Just thought you ought to know after the fact."
That was enough for Celestia to crack an eye open, the better to give her sister the sort of look she usually reserved for overzealous students. "Luna, we're trying to be diplomatic."
Luna answered with the sort of look she usually reserved for ponies who dreamt of the diarchs in compromising positions. "They were trying to ignite our atmosphere."
"Ah." Celestia gave a single nod and shut her eye. "I'll have to tell their Emperor. We've been having such a lovely chat as I help him pull himself together; I can't imagine he'll be happy with the idea."