As virtuous men pass mildly away,
     And whisper to their souls, to go,
Whilst some of their sad friends do say,
     "The Breath goes now," and some say, "No":  4

So let us melt, and make no noise,
     No tear-floods, nor sigh-tempests move.
'Twere profanation of our joys
     To tell the laity our love.                                         8

Moving of the earth brings harms and fears,
     Men reckon what it did and meant;
But trepidation of the spheres,
     Though greater far, is innocent.                         12

Dull sublunary lovers' love
     (Whose soul is sense) cannot admit
Absence, because it doth remove
     Those things which elemented it.                       16

But we, by a love so much refined
     That our selves know not what it is,
Inter-assured of the mind,
     Care less, eyes, lips, and hands to miss.            20

Our two souls therefore, which are one,
     Though I must go, endure not yet
A breach, but an expansion,
     Like gold to airy thinness beat.                          24

If they be two, they are two so
     As stiff twin compasses are two:
Thy soul, the fixed foot, makes no show
     To move, but doth, if the other do.                     28

And though it in the center sit,
     Yet when the other far doth roam,
It leans, and hearkens after it,
     And grows erect, as that comes home.              32

Such wilt thou be to me, who must
     Like the other foot, obliquely run:
Thy firmness makes my circle just,
     And makes me end where I begun.                    36

     "Moving of the earth" in line 9 refers to an "earthquake." "Trepidation of the spheres" in line 11 refers to a major problem in astronomy at the time. At that time, people still believed in the Ptolemaic system of the universe -- that the earth is at the center and that the sun and all the planets and stars circle the earth on concentric rings. It had long been possible to predict where in the sky the various planets would appear, but the development of the telescope in 1608 had revealed planets either slightly ahead or slightly behind where they should have been. This was referred to as "trepidation of the spheres" probably on the assumption that the spheres appeared to be vibrating. [Consider what happens when you snap a string tight. As the string vibrates, it appears to be in two or three different places, one forming an arc above, another an arc below, the central line of the string.] 
     Donne considers this trepidation to be "greater far," i.e., far greater in importance than an earthquake because the entire political and religious systems of the time were entrenched in the Ptolemaic system of the universe. The pope and Kings, for example, were associated with the outer "ring" in the Ptolemaic system (the ring closest to God). Trepidations could cause the entire universal system to collapse, which, of course, it did. [Martin Luther had published his Ninety-Five Theses in 1517, and the Protestant Revolution was already well under way. The collapse of the "scientific" support of the Catholic Church (the Ptolemaic concept) was thus extremely important. When Donne says that it "is innocent," he means that it is "innocent" to the "men" in line ten. These men respond only to physical stimuli (like earthquakes). In a sense, we may be seeing the same thing going on today with genetic cloning. Most people, like the men who only respond to earthquakes, pay little attention to the question of cloning. But if humans do start to clone humans, the religious implications will be extreme -- does a clone have a soul?
     "Sublunary," in line 13 means "under the moon" and refers to the same Ptolemaic system. The belief was that matter (the physical universe) fell toward the center -- the earth. The outer rings of the universe were considered to be more spiritual. Note how lines 13 to 20 develop this idea.
     The reference to gold in line 24 implies the belief at the time that gold could infinitely be pounded thinner and thinner and would never break.
     The "stiff twin compasses" in line 26 refers to the compass that we use to draw circles. Note how this metaphor is developed throughout the rest of the poem.

Freccero, John. "Donne's "Valediction: Forbidding Mourning" ELH, Vol. 30, No. 4. (Dec., 1963), pp. 335-376. [JSTOR] [R3]