I'm including here a link to a thread I started on the subject some time back. Some of it may be of interest to you. Also, Virella's summary she linked above is handy.
Others have supplied good answers to your Sharlayan-related inquiries, but to clarify your third and fourth questions:
3) We are introduced to as many cards of the Major Arcana as there are Heavens' Gates (also called constellations, as others have noted). Not all constellations house Heavens' Gates, I'd imagine, but so far all the Heavens' Gates are constellations (although there is a Seventh, which I can't remember, and which doesn't seem to have a corresponding card). It is possible, likely even, that there are other Major Arcana beyond the six we have mechanically, and it is certain that there are any number of Minor Arcana that we don't know anything about specifically, but are mentioned in quest text.
4) I would guess the Star Globe/Planisphere/Astrolabe could be used for all the same purposes as real-world star globes, planispheres, and astrolabes. Navigation, chiefly, as well as calendar and date calculations and astronomical observations. I don't think it's just a magical focus, but an actual instrument for ascertaining optimal aetheric flows from the stars based on their locations as determined by proper application of said instrument.
^my two gil
Others have supplied good answers to your Sharlayan-related inquiries, but to clarify your third and fourth questions:
3) We are introduced to as many cards of the Major Arcana as there are Heavens' Gates (also called constellations, as others have noted). Not all constellations house Heavens' Gates, I'd imagine, but so far all the Heavens' Gates are constellations (although there is a Seventh, which I can't remember, and which doesn't seem to have a corresponding card). It is possible, likely even, that there are other Major Arcana beyond the six we have mechanically, and it is certain that there are any number of Minor Arcana that we don't know anything about specifically, but are mentioned in quest text.
4) I would guess the Star Globe/Planisphere/Astrolabe could be used for all the same purposes as real-world star globes, planispheres, and astrolabes. Navigation, chiefly, as well as calendar and date calculations and astronomical observations. I don't think it's just a magical focus, but an actual instrument for ascertaining optimal aetheric flows from the stars based on their locations as determined by proper application of said instrument.
^my two gil
