How does Jackson use Earth's point of origin when dialling off Klorel's ship?

In Stargate SG-1 episode 2.01 The Serpent's Lair, Daniel dials off the ship to the Alpha Site, and later explains that he was close enough to Earth to use Earth as the point of origin. But how does he do that exactly? It's not like the DHD on the ship would have an Earth point of origin symbol (sun over a pyramid).