This might be confusing, since the home of the current seat of Triconian power is also named Earth. A little known bit of information might help explain here: when Jason Blackthorn's men captured Alpha Virginis II, their archeologists found millenia old maps of the Galaxy. There was a planet named Earth on the other side of the galaxy from where they were. Correlating from where thier home planet was, they found reference to a little ancient Triconian outpost named "Urth". Urth? Was this really the name of the planet 5000 years ago?
After first contact and a limited amount of development in intergalactic space, the Triconians and everyone on Urth was subjugated by a roving, extra-galactic species known as the Armex. This decimated the surface of the planet and drove the population to hiding underground. The Armex were vanquished when a returning Triconian explorer brought a captured ship more powerful than all of the Armex resources in the sector. This "miracle-ship" was of DeathSmith origin.
Deathsmith technology was kept off of earth since it was far too advanced for Urth scientists to understand. The Urth was left to fend for itself again, to grow more powerful with its own might.
After the Armex were vanquished, the Triconians promptly rebuilt their space fleet with some help from surrounding planets. The then explored the sector further and discovered the Cyran Empire, shortly thereafter, Cyrus' Star.
Standardly, Triconians refer to Urth as Earth, and that will probably be the spelling you'll see it as most.
During the exploration trip, the Dur Zeppelin discovered the Orisian Empire, and promptly got into a small spat with them when on their way out of the empire. This battle was over an ancient Triconian world named Alpha Virginis II. With many, many Orisian deaths on Blackthorns hands, he takes the planet and promptly starts trading with the local neutral planets. By now, he has most of the neutral planets eating out of his hands.
Only recently, within the last five years, have the Blackthorn Triconians started the process of restablising communications and long-distance trade with Earth.