When they re on the moon and decide to go to Earth to rule, as Zod says. What would ve happened if they d have been say on a moon of Corruscant, with The Emperor and Vader down there, i doubt they d have "ruled"
if you stay on the moon the energy cannot pull you without the power of the red sun so you cannot rule the emperor would fall and chaos would befall the galaxy and it could mean the end of man kind.