[Banestorm] How little can a resident of Yrth know about the setting?

[Phil Masters]

Yes. The original GURPS Fantasy version of Yrth had different constellations, but after GURPS 4th ed. made it effectively a parallel Earth with different geography, we decided that was far too much of a can of worms, and quietly put the stars back where they belonged.

[Michael Cule]

I would say how much knowledge a person has depends on:

Where they are and who they are socially. (Mostly to decide what levels of default skills are likely. A noble will have picked up a totally different set of causual knowledges than a peasant.)

What actual skills and advantages they have. Cultural familiarity is important as mentioned above. Actual skills that have implications for knowing the world background include:

History
Literature
(The sagas of the founding of the early kingdoms will be damned important for what most people know and there may be some confusion with imported figures from Earth history and myth being incorporated into Yrth's stories. Perhaps Simon Menaleus found it wise to claim descent from Caesar or King Arthur.)
Area Knowledge and Geography
Theology.

Personally, I think you're on a hiding to nothing keeping the fact that the game is set on Yrth from the players unless you either go for Sahud as the setting or somewhere on one of the unexplored continents. Trust your players to keep player and character knowledge separate and just tell the tale.

[Astromancer]

Originally Posted by Miles

Yup, the astronomy is almost the same. Besides the mana-asteroids, the moon looks different, with a brighter, more silvery glow to it, and an entirely different pattern of craters is totally different. But it's the same size and in the same place, just like the sun, stars, and planets.

Since most pre-modern people were very familiar with a naked eye view of the moon, a radically different arrangement of craters would be a dead give-away, of a sort.

I could see Yrth people thinking that Earth is on the other side of the Moon and that the people on Yrth were seeing the farside of the Moon.