MY Love is of a birth as rare
As 'tis, for object, strange and high ;
It was begotten by Despair,
Magnanimous Despair alone
Could show me so divine a thing,
Where feeble hope could ne'er have flown,
But vainly flapped its tinsel wing.
And yet I quickly might arrive
Where my extended soul is fixed ;
But Fate does iron wedges drive,
And always crowds itself betwixt.
For Fate with jealous eye does see
Two perfect loves, nor lets them close ;
Their union would her ruin be,
And her tyrannic power depose.
And therefore her decrees of steel
Us as the distant poles have placed,
(Though Love's whole world on us doth wheel),
Not by themselves to be embraced,
Unless the giddy heaven fall,
And earth some new convulsion tear.
And, us to join, the world should all
Be cramp'd into a planisphere.
As lines, so love's oblique, may well
Themselves in every angle greet :
But ours, so truly parallel,
Though infinite, can never meet.
Therefore the love which us doth bind,
But Fate so enviously debars,
Is the conjunction of the mind,
And opposition of the stars.
This map from 1650 is probably the type of cramp'd space that Marvell intended in using the term "planisphere" -- the spherical earth shown in plan, two-dimentionally, with plentiful parallel lines (and a few oblique).
We usually think of planispheres as star maps like this showing the heavens above the Northern Hemisphere in flat plan with lines showing geometric relationships of the stars: plentiful lines oblique, but none parallel. Its possible Marvell had this in mind when writing the last stanza.
This one comes from a late 18th-century update to John Flamsteed's celestial atlas. Flamsteed was Royal Astronomer whose life overlapped Marvell's but he hadn't published anything by 1652 when this poem was written. (Flamsteed's dates are 1646-1719.)