This quest, while technically part of the 2nd phase of the Shattered Sun Offensive, does not appear immediately at the beginning of the Offensive. Instead, your server must complete the portal from Quel'danas to Shattrath by doing the Intercepting the Mana Cells quest.
Once you've unlocked the portal, a new NPC, Astromancer Darnarian, will be found inside the Sanctum building alongside the other daily quest givers. When you talk to him, he will complain that no-one appreciates the effort that it takes to do Translocations, and demands that you educate yourself by taking an Astromancer's Crystal and attuning it to 3 different places: The Bloodcrystal to the west, the Dawning Square portal to the south, and the naga shrine at Greengill Coast to the southeast. Be sure to use the crystal at each area to get your credit.
The Bloodcrystal can be found due west of the Sanctum, at about location 42,35.
The Greengill Coast point will take a bit of riding, it's on the east coast of the Island, about halfway down the length of the island, at about location 61,62.
The red dots on the map below give the approximate locations of each point, or you can click the map for a link to a larger map by Wowhead user Norrec:
Once you've gotten your readings, return to the Astromancer. You'll get 11 gold 59 silver and Darnarian's Scroll of Teleportation, which will send you back to Shattrath City when you use it on the Isle of Quel'danas. It's only good for one use, but fortunately, you can do this quest to get a new one every day. Unfortunately, it's also unique, so you can't just hoard the scrolls for a rainy day. Still, it should at least be useful for heading back to Shattrath should you need to repair until the Sun's Reach Armory is retaken or get started on the new Shattrath dailies. Not bad for a bit of riding. If you need more information on this quest, check Wowwiki and Wowhead.