To unlock Astrologian, you must gain access to the city of Ishgard by completing the final Main Scenario Quest of A Realm Reborn, 'Before the Dawn.' The Astrologians' guild (the Athenaerium Astrologicum) can be found in Ishgard in The Pillars (X:15.2, Y:10.0). Accept the quest 'Stairway to the Heavens' from Jannequinard. Upon completing the quest, you will unlock Astrologian, which starts at level 30.