"...that is. An interesting application for time travel. Why is it dangerous to interact with one's past self, is it too tempting to destabilize your time loop and stuff explodes or what? Wouldn't that risk apply to existing in the same general area at all?"

"Entirely too many wizards, upon observing their past selves, attempt to interact with them in ways inconsistent with their own memories of the other perspective. Since a single consistent timeline must always result, their attempts to create contradictions necessarily fail. If someone on the scene is sufficiently quick-witted, consistency can be preserved with memory modification; however, in many cases events turn out to be consistent by preventing the interaction at all, often injuriously to the time traveller or nearby people and scenery. Are you familiar with Muggle accounts of the 'Tunguska Event'?"

"Well, yes, though on my Earth it was in fact just a meteor. I suppose something like that has to be going on to keep the timelines recognizably similar though it beats me how..."

"Yes, though I'm puzzled why the timelines would be similar at all. It is admittedly convenient."

Vector chimes in: "Maybe there are loads of timelines and whatever caused Swan to end up in the wrong one can only go between similar ones. So it's like asking why the lottery's numbers and the winner's numbers happen to be similar."

Flitwick: "What's a lottery?"

Vector: "Muggle way of robbing each other, don't worry about it."

"Oh, come on, I'm not interested in defending the lottery but it's gambling, not robbery, haven't you lot got gambling?"

Vector: "Only bets between individuals, not massive things like the muggles have, but fair enough, I was being glib."

Flitwick: "The Daily Prophet had a big drawing sort of thing this summer."

Vector: "Oh, did they? Alright. Anyway, when you're an arithmancer all 'games of chance' look like complicated ways of robbing each other no matter who's doing them."

"In terms of expected monetary value, absolutely, and some people get dangerously in deep to any form of gambling, but it's usually the done thing to construe it as people paying for the experience of having a chance, small but not in a way that's easily legible to the average person, of winning big. A lottery ticket is a fantasy-construction aid, as it were - in its most responsibly played form, which of course is not the overwhelming use case."

"Huh. You learn something new every day. Say, I've always wondered how much Arithmancy it's possible to do without magic, I mean my kind of magic. How about I give you some lessons this year, whenever we're both free, just to see how it goes?"

"Sounds great! I don't sleep unless I'm super bored or something so I hope to have plenty of time. Is arithmancy importantly different from just... math? Arithmetic, algebra, calculus, all that jazz?"

"A lot of it is just math! Probability, algebra, tensor calculus when things get really interesting. And then some of it's magic. It's a bit like how potions is just cooking, but also magic, but the boundaries are even harder to define."

"I was assuming that the ingredients were magic, or that magic was done to them? I'm definitely confused about how you do magic to math, though."

"The ingredients are often magic, but a muggle with the right ingredients following the right recipe wouldn't get the right results. I suspect a similar phenomenon is at work in arithmancy: many divinatory processes call for a random number or a gut estimate or a measurement taken with an enchanted device. But spell development in particular, to the best of my ability to determine, is purely nonmagical though rather complicated math applied to magical subject matter, such that the only difficulty to be expected would be in testing one's hypotheses. Which is also the hardest step for wizards, albeit for different reasons."

"Huh, so I could do spell development if I wanted?"

"Unless there turns out to be a magic step I don't know about, yes! Except for the part where you find out if what you've made is a spell to blow yourself up."

"I'd have to outsource that, which is a real shame, I'd cope better than most."

"Ironic, that." Vector joins in the Quidditch chatter for the rest of the way to the Moon.

It's a dumb game but Cam feels that way about most sports and doesn't feel the need to weigh in.

Moon!

Fascinating. (She conjures a steady supply of air.)

Air supply and then Wheeeeeeee boing boing boing.

(Sinistra is holding perfectly still and possibly having a religious experience.)

. . . Cat form! Wheeeeeeeeeeeeeeeeeeeee boing boing boing. Pity all those who have never seen the fabulousness of a humanly intelligent cat showing off in one-sixth gravity.

That's so cute but he will politely refrain from videotaping it. He makes air continuously as it disappears into the vacuum. Sits in the moon dust and makes mooncastles.

Eventually it's time to be an extremely dignified witch again. Everyone finishes up and heads back to the shuttle. 

"Thank you for the excursion; it was both pleasant and edifying. I have no objection to your bringing your NEWT students here in the future."

Wag wag. "I look forward to it. How many kids are in that level of the program now?" he asks, directing the question to Charity as the shuttle lifts off.

"Ten seventh years and eight sixth years. The class of '95 is getting into the last war's birth crunch and, well, it was never a popular class. Maybe you'll be able to change that in time to pick up the baby boom."