lemoncatpower
Cynical Optimist
Again, Solgaleo and Lunala can't control the sun and the moon, so they can't cause eternal day, night, or an eclipse. That and Zygarde protects the eco-system in general, and him being in Alola does imply something must be wrong over there.
I don't believe it's confirmed that they don't have any sort of control over the celestial bodies.