Huh. Is your sun about as big in your sky as ours?

Can't look straight at it obviously but I haven't noticed it being different either way. Stars looked plausible too. You have so little light pollution here, I bet if I went for a hike in the dark and managed not to break an ankle the stars'd look incredible but even from the house I can see a lot.

There's an observatory up in the mountains. It takes about a day to get there. The air's thinner so some things are easier to measure, and you're notably closer to both the sun and the moon.

I don't... think a day's hike gets you notably closer to the sun or the moon on Earth. That'd be like a one digit number of miles? I think Everest's like four miles, so about that much even if you hike really fast and the mountain's very climbable. And the moon is like... millions of miles away, I think a bunch of Earth-diameters all in a row, though I don't remember figures. You'd never notice being four closer though sometimes you notice when the moon's at a closer point in its orbit. The air thinning part is the same though.

I am relieved to discover an observable difference between worlds that work so differently.

And the Sun's way farther than that! Earth is the third planet in the solar system, the other planets are much farther away than the moon, and even the one closest to the sun is not that close. She sends her vague mental image of the Solar system.

How does gravity work?

I don't know the math of it and obviously you will not be able to rederive it here but everything is pulled toward literally everything else - uh, unless gravity waves are in fashion among physicists lately? I don't know, it's never been obvious to me if it's supposed to propagate instantly and if it doesn't then it shouldn't be literally everything else, just stuff close enough - uh, things are pulled to other things, more the more massive the things are, which is importantly distinct from how heavy they seem while you're standing around on a planet, and more the closer they are. That's why our air thins out, the air is pulled to the earth. If you go high up enough there's none.

Why aren't the planets pulled towards the sun?

They are! They just keep missing! ...I do not know enough orbital mechanics to explain why. I remember they go in ellipses. Moons also work this way with their planets.

He pulls out a piece of paper and a clipboard and a pencil. How long would it take to sail from one end of your world to the other?

Uh.

- that's for circumnavigation. It's a sphere.

A sphere. 

He is going to be rude, now, and he tries very hard not to do that, but this is important. She can walk on the surface of their world without very much trouble, so the pull of gravity on both surfaces must be comparable. (Well, she has a little bit of trouble. Maybe her world's pull is a little less.)

Gravity is a relationship between objects, affected by their size and how distant they are. Are any other things relevant? Assume not, see if anything else is required to make the whole thing work, later. How does their size matter? Assume linearly, to start, why not? How does distance matter? Assume linearly, to start, why not? - no, it's obvious why not, you wouldn't have lots of distinct objects that way. A universe where the effects of distance are linear is flat, in a sense, so it ought to fall with distance squared -

Then there's only a constant required to make the whole thing work, though the numbers that make it work are all eye-popping. 

He looks up after - he's not really sure how long, actually. - sorry.

She's been working on a little lesson plan for the linguists for tomorrow once she realized he was a bit far away. "It's okay."

"I try - really try - not to."

"...not to what, get nerdsniped?"

"That's a compound of -"

"'Nerd' is people who think academic type stuff such as deriving the law of universal gravitation is lots of fun, and 'snipe' is to hit with a ranged weapon but in this case it's metaphorical."

"The thing I try not to do is value questions more than people or courtesies or duties. I realize that doesn't sound like it should be very hard but -"

"I have some idea of how exciting all this might be, I don't have a particular need to seem more interesting for my winning personality than for my ability to dispense high school science information."

"I think it's a tendency many people can indulge safely, probably."

"Look, why do you think I'm not being like, 'aaaah, I have to get home, my parents are worried' - it's because I know important stuff you guys don't have here! It matters! It's worth caring about!"

"So -

Everything we have, here, we have because we invented it. Understanding the world matters, inventing things matters, it's the reason we have food and shelter and safe roads and good records and good medicine and effective artifacts, and if we ever - if we're sure it's a good idea to fix the thing where we will eventually need to return to Valinor, and if we fix it, it'll be by understanding, it'll be by inventing, that's fine, it's fine to hold those things sacred for that reason -

- but if something is beautiful enough some people can forget that it matters because it helps people and will no longer matter if it no longer helps people, and those people need to be careful, because their hearts are not very good guides."

"...okay, but you don't have to apologize to me for getting nerdsniped by gravity."

He chews on his lip for a second, catches himself at it, stops.

"I should - probably finish the story."