Mary Geoise Rulers/Inhabitants are called "Celestial Dragons" because They represent Celestial Bodies & They believe that Earth is Center of Everything, meaning there is a Character who represents Earth that They worship
Gorosei were chosen as Highest Authority because They are like this God's Priests (And it's represented by their Names that are based on Five Planets closest to Earth, based on Ancient Knowledge ofc, not Modern), and Imu was given a Seat even above them because He is this God's Grand Priest and He represents Moon which is Closest Celestial Body to Earth
Then there is a Sun Character who wasn't meant to be above Gorosei in Authority or their Equal, but He was meant to become Highest Authority after them (Probably Garling's Position)
However, this Character ended up outside WG because He is supposed to replace Earth instead, because Planets revolve around Sun, He is basically Character who isn't just meant to sit on Empty Throne, He is supposed to be God of Celestial Dragons
And Oda already Introduced us a Character who is a Former Celestial Dragon, who was supposed to Replace Garling, who have Divine Presence/Attacks, who associates himself with a Dragon, it's none other than Shanks
So Imu is "Moon Character", Gorosei are "Five Elder Planets" and Shanks is "Sun Character"
Who is "Earth Character"? It's someone who isn't Related to World Government at all, but is actually Host to their God and Shanks Nemesis, It's Blackbeard (He represents Sea Devil, and that's what WG is, They are Church of Satan)
As for Luffy, Yes He also represents Sun, but He isn't World Government's Sun, He is Elbaf's Sun
But this is a Story for another time