Moving of th' earth brings harms and fears, Men reckon what it did, and meant;
"Moving of th' earth" is a funny way of saying "earthquakes." Earthquakes were (and still are, really) pretty unexplained phenomena. Donne refers to them, though, to emphasize their violence—earthquakes bring "harms and fears." They shake everyone up and make them wonder "What the heck just happened?"
Line nine begins with a reversed iambicfoot, called a trochee. Instead of the expected iambic rhythm (da-DUM), we get "Moving." This is pretty common, especially at the beginning of a line, but the fact that this is the first reversed beginning might be Donne's way of putting extra emphasis on the violence of the earthquake. By opening the line with a stressed syllable, it packs a little more energy.
Notice that line 10 moves us from the earthquake to peoples' reactions. Donne reminds us that all this talk about natural disasters is just a long-winded explanation of why it would be wrong to make a big show over his departure. Here, he is saying that earthquakes cause mass confusion and panic.
But trepidation of the spheres, Though greater far, is innocent.
You can't go too far in a seventeenth-century poem without some reference to the cosmos. So fine, let's get a quick primer on all these celestial references. Everything above the earth moved in spheres: the moon, planets, the stars and sun. The spheres were concentric—picture those Russian nesting dolls. Those spheres moved in their own patterns, but different motions, vibrations, and alignments created what they referred to as "celestial music" and that divine symphony controlled everything in the universe—from the creation of planets and stars to what you are going to eat for breakfast.
Now "trepidation" usually means to be afraid or anxious, but this older meaning actually means to make a literal trembling motion. So Donne is referring to the trembling motions and vibrations of the heavenly bodies.
Hmm. Looks like we may be off again on another metaphor. This one may take a while to unpack—stick with us.
The two important points of the metaphor are in line 12. First, the motion of the spheres is "greater far." Yeah, sure, earthquakes make a big scene down here on our little rock, but we are talking about the freaking universe here, folks. An earthquake might shake stuff up for a little while, but the motions of the spheres controls all eternity—much bigger deal.
The second point in line 12 is that these motions (unlike the earthquake) are "innocent." Not innocent like guilty/innocent, but innocent as in unseen or unnoticed. We are innocent of these all-powerful forces. Earthquakes are all show, but the motions of the stars are subtle, quiet. Earthquakes, in other words, are shallow.