User blog comment:Miskos3/Fairy Tail Chapter 367: Jellal vs. Oracion Seis, Review/@comment-4449950-20140121084703/@comment-5303512-20140122000754

Think of it this way, the planet revolves around the sun which would be considered moving, think of the Tartarus HQ as like a object revolving around another object, it is probably also because it has it's own gravitational pull it is un effected by motion.