The poem is a string of odd, shifting images for an eclipse, where the sun is no longer the bright center of the sky. It starts with a startling pairing: the Lord as a lamb, gentle and small, while the sun is "nothing." The rest of the poem tries to say what that nothing looks like, and it never settles. The sun becomes a sheep's head, then a "war bark," a "public hatch," "sad money," and finally a hand with a high fever.
The speaker doesn't explain these comparisons, and that gives the poem its unsettled mood. Some images feel bodily and sick, like the feverish hand. Others feel like loss or threat: the barking of war, money that seems mournful. The speaker seems unnerved, as if a familiar source of light and order has gone missing and the mind grabs at whatever it can. Judging by the title, the darkened sun is the thing being described, and the poem leaves us in that strange, uneasy dimness.