trepidation+of+the+spheres

Trepidation is a feeling of fear or agitation about something that may happen; or a trembling motion.

"trepidation of the spheres" has to do with Ptolemaic theory of the universe (all the planets had their own sphere, with Earth being the center sphere)

The use of this statement in the third stanza compares Donne's vision that the speaker's love is unlike an earthly love.