Yu-Gi-Oh! » Rulings Q&A

Opponent summons The Winged Dragon of Ra - Sphere Mode to my field. Next turn, I tribute summon using it as a tribute. Is that a legal move?

Yes, you can use Sphere Mode as a tribute

Yes you can do whatever you want with a Sphere Mode on your field, except for attacking with it. It only has restrictions (targeting and attacking it) for the opponent of whoever controls it.