I'm still working through AST myself (currently 56) and my grasp on the details may not be airtight, but I'll try to answer where I can as best I can!
1. Astrology has been practiced on Hydaelyn for a while - though I couldn't say for how long. One form of it has been used for a while in Eorzea itself - Ishgardian Astrology - specifically to gather information on oncoming assaults of the Dravanian Horde. The type the PC uses - Sharlayan Astrology - is more battlefield oriented and involves channeling the power of the stars to both heal and harm rather than just foretell the future. Not only has it been considered less useful by the Ishgardians in comparison to their own... but Sharlayan itself is surprisingly against divulging knowledge in certain cases such as this.
2. If it was mentioned who actually started Astrology, I either haven't seen it or missed it entirely. Considering the depth of power difference, though, I would likely assume it was the Sharlayans.
3. The six cards are based on the six constellations, and you draw upon the powers of those star formations through your Astrology to shift the tide of battle. I don't see any reason why there couldn't be more cards, but unless they're paired with another constellation of power (which I don't believe there are any more of) they'd be little more than just cards.
4. I would assume the astrolabe is being used to plot the location of the various constellations in the heavens, so you can focus on them to draw upon their power through the cards. While that's its main purpose, I could assume one could use it for similar things you would do with anything that involves the tracking of stars - such as one's location on Hydaelyn.
1. Astrology has been practiced on Hydaelyn for a while - though I couldn't say for how long. One form of it has been used for a while in Eorzea itself - Ishgardian Astrology - specifically to gather information on oncoming assaults of the Dravanian Horde. The type the PC uses - Sharlayan Astrology - is more battlefield oriented and involves channeling the power of the stars to both heal and harm rather than just foretell the future. Not only has it been considered less useful by the Ishgardians in comparison to their own... but Sharlayan itself is surprisingly against divulging knowledge in certain cases such as this.
2. If it was mentioned who actually started Astrology, I either haven't seen it or missed it entirely. Considering the depth of power difference, though, I would likely assume it was the Sharlayans.
3. The six cards are based on the six constellations, and you draw upon the powers of those star formations through your Astrology to shift the tide of battle. I don't see any reason why there couldn't be more cards, but unless they're paired with another constellation of power (which I don't believe there are any more of) they'd be little more than just cards.
4. I would assume the astrolabe is being used to plot the location of the various constellations in the heavens, so you can focus on them to draw upon their power through the cards. While that's its main purpose, I could assume one could use it for similar things you would do with anything that involves the tracking of stars - such as one's location on Hydaelyn.
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