John Donne (1573-1631)

Mourning: anxiety, worry, apprehension.
A Valediction Forbidding Mourning
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:
So let us melt, and make no noise,
No tear-floods, nor sigh-tempests move;
'Twere profanation of our joys
the laity: all persons who are not priests or in religious orders.
To tell the laity our love.
Moving of th' earth: earthquakes.
Moving of th' earth brings harms and fears,
Men reckon what it did, and meant;
trepidation of the spheres: i.e., sky-quakes, irregular movements of heavenly bodies. See the Wikipedia article on Trepidation (astronomy).
But trepidation of the spheres,
Though greater far, is innocent.
sublunary: earthly; literally, below the moon.
Dull sublunary lovers' love
admit: allow or welcome into.
(Whose soul is sense) cannot admit
Absence, because it doth remove
Those things which elemented it.
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.
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.
If they be two, they are two so
stiff twin compasses: draftman's compass, used in charting maritime navigation. Despite the plural "compasses," it's just one object, as a "pair of pants" is just one object.

As stiff twin compasses are two;

Thy soul, the fixed foot, makes no show
To move, but doth, if the other do.
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.
Such wilt thou be to me, who must,
Like th' other foot, obliquely run;
just: accurate, perfect, as in a justified margin.
Thy firmness makes my circle just,
And makes me end where I begun.