How does the sky know where I am?

It doesn't, exactly — and that is the point. RIPEN never asks for your location. It reads your time zone, which the phone gives up without a permission dialog, and that turns out to be enough for a sky you would recognise.

what is actually up there

The tree stands under a real sky. Sun and moon follow your real date and hour, the seasons turn, and above them stand 5,044 stars — every star down to about the faintest the naked eye can see on a dark night. They are not scattered for decoration. Each one sits where it actually sits tonight, from where you are.

172	stars down to magnitude 3 — the ones you can pick out from a city
5,044	down to magnitude 6 — what a dark sky gives you

Each star carries its own measured colour, so Betelgeuse is warm and Rigel is blue. The Milky Way lies along the galaxy, at the angle your latitude and hour put it. And the planets do not twinkle — a planet shows a small disc rather than a point, and the air cannot wobble a disc the way it wobbles a point. That is how you tell one from a star with your own eyes, and it is the same in the app.

why no permission

A location permission would have been the easy way to get this. We did not want it, for a reason that is not really about stars: RIPEN has no account, no analytics and no requirement to be online. Asking where you are, in order to place a shadow correctly, would have been the loudest contradiction in the whole app.

So the sky is built from what the phone gives freely — your time zone.

what a time zone knows

A time zone is not only an offset from UTC. It has a name, and the name is a place: `Europe/Warsaw`, `America/Sao_Paulo`, `Asia/Tokyo`. The identifier is built around a city, so it carries roughly where that city is — which is what a sky needs.

If the zone is one we recognise, the app places you near its city. If it isn't, it falls back to the zone's own meridian, which an offset alone is enough to work out. Either way nothing is asked and nothing leaves the phone.

where it stops being true, and we would rather say so

A time zone is a wide thing. `America/Chicago` covers a lot of ground, and its city is probably not your city. So:

the sky is right for your region, not your street. Is Orion up, where is the moon, is that bright one a planet — those hold. Anything needing arcminutes does not;
latitude is the weaker half. Longitude mostly decides what time the sky thinks it is; latitude decides what is above the horizon at all. If we don't recognise your zone, latitude is a guess, and a guess that is twenty degrees out puts the wrong constellations overhead. That is why you can set latitude by hand in the app;
the clock moves, the planet doesn't. Daylight saving shifts your offset twice a year without moving you an inch, so the sky is built from your standard offset instead. Get that wrong and every star rises an hour early for half the year.

None of this needs your consent to a permission dialog, because none of it is your location. It is the shape of the time your phone already keeps.