Back in the day, Sonic lived on Mobius and Robotnick was one of the only humans living there. I left the Sonic universe for a couple of years *slaps hand, but when I returned to SA2:B on the GC, it seemed Sonic was on Earth, there was no mention of Mobius and they even call it Earth I think in a few cut scenes. Where is Sonic now and if it is earth- is there any explanation as to why he's there or is it a new storyline?
In Japan, the series has always taken place on Earth, and the world was dubbed "Mobius" in the translations. The games recently adopted the original names (hence the change of Robotnik to Eggman) so now, in the SEGASonicverse, it's Earth, while in the Archie/SatAM/AoStH/SU versions, it's Mobius.

~ Crazy C Lea