Nobody here calls the moon a silver lantern or a pearl. The speaker scolds it like a teenager who won't come in off the street. "Gibbous" names the phase between half and full, when the moon is more than half lit but not yet round. Over eight lines the poem tries four insults on that awkward shape, then turns the joke on the poet himself.
The opening couplet sets the tone with "Shameless procrastinator." Part of the logic is astronomical. Past full, the moon rises later each night, and a gibbous moon can still hang in the morning sky when it ought to have gone. The rest is character. The moon lingers, and it feels no guilt about it. The second line, "ragged tooth unsullied by the dawn," holds two opposed ideas. A ragged tooth is decayed and uneven, but "unsullied" means pure and untouched. The moon is shabby and spotless at once, which suits a creature too brazen to be embarrassed.
The second couplet goes on with the household comedy: "Full, you went to bed on time; / a quarter empty & you never act your age." A full moon is complete and well behaved, and the gibbous one is that same moon gone a little loose. "A quarter empty" borrows the language of a glass or a petrol tank, and the ampersand is a casual scribble in the middle of the line. It makes the speaker sound like someone muttering at a younger relative. The poem's astronomy is playful rather than strict, but the joke about age works. We expect the older moon to retire early, and this one stays out later and later.
The third couplet changes the metaphor from a person to a thing. A hasp is the hinged metal strap that fits over a staple and holds a door or gate shut. The gibbous curve could pass for one, and it has "no padlock," so nothing is secured. "No wonder the night got away!" The night becomes an animal that has slipped out of an unlatched pen, and the moon is the careless keeper who let it go. This is the poem's only exclamation mark, and it sounds like grumbling that has turned into delight.
Then comes the bluntest image, "Old flat tire." It is only three words, and the line itself seems deflated. The moon has lost its roundness along one edge, and the language has dropped from the heavens to the garage. There may also be a faint pun, since "tire" sounds like "tired," which fits a moon that has been up too late.
The final line changes the poem's direction: "As if my poet's O were set in gothic." Here the speaker enters for the first time with "my." The "O" is both the letter and the old lyric gesture of calling out to the sky, as in "O moon." A round, clean O is the natural shape for such an address. Set in gothic type, with its angular, broken strokes, that O would be squeezed out of true, and the moon's lopsided outline looks like exactly that. "Gothic" also brings in the shadowy atmosphere of night. The joke has been aimed at the moon all along, but here it lands on the poet's own tradition of grand address. The moon cannot manage a perfect O, and neither, the poem admits, can the poet.
The poem is built from four unrhymed couplets, each holding one thought, and none of them runs over into the next. The lines are uneven and close to speech, with hard consonants, such as the "t" and "g" in "ragged tooth" and the "h," "s" and "p" of "hasp," that clip the phrases short. Apart from the closing "As if," the images are bare metaphors. The moon is a procrastinator, a tooth, a hasp, a tire, and the speaker never hedges with "like."
The poem matters because the moon is one of poetry's most worn-out subjects, and it renews it by picking the least photogenic phase and refusing to be reverent. The humour is affectionate, though. The speaker pays close attention to the shape in the sky, and the poem's last move is to admit that the poet's own lyric voice is as imperfect as the thing it addresses.