The title does quiet, heavy work. "Sublunary" means, literally, "beneath the moon." Older pictures of the universe used it for everything below the moon's orbit: the changeable, mortal, decaying part of creation, as opposed to the unchanging heavens above. John Donne's "sublunary lovers" is probably its best-known appearance in English. Before the poem's first line, then, we already know we are on the side of the sky where things wear out. What follows is only twenty-odd words, arranged in three couplets, and they turn that idea into something small, human and a little wry.
The opening couplet, "this is not weather / for the young," is slippery in a productive way. "Weather" might be literal, or it might be the climate of a whole stage of life. The sentence doesn't say whether this weather is too harsh for the young or simply too slow and quiet for them. The phrasing may remind some readers of Yeats's "That is no country for old men," but here the exclusion runs the other way, and the tone is calmer. Notice too how "for the young" sits alone on a three-syllable line, so the young are literally pushed to the edge of the poem.
In the middle couplet, two people appear: "you believe in the moon / and I the sun." The verb "believe" is odd. Nobody needs faith to know the sun exists, so the word suggests allegiance or temperament. Judging by the imagery, one person leans toward night, dreams and reflected light, the other toward day, heat and plain sight. The second line drops its verb entirely, "and I the sun," trusting the reader to carry "believe" over from the line before. That shortening feels like intimacy, the way people who have known each other a long time finish each other's sentences. The poem never says whether "you" and "I" are lovers, friends or something else, and it doesn't need to. They disagree gently and have clearly learned to live with it.
The last couplet dissolves the disagreement. "While we turn old / and rest day and night." The pronouns have been moving all along, from the general "the young" to the separate "you" and "I" to a joined "we." Age is the thing that brings them together. The verb "turn" is well chosen. It is what the earth does to produce day and night, what the moon does in its phases, and what happens when milk or leaves or fortunes change. Growing old becomes another rotation, one that happens to both of them whatever they believe in. And "rest" is deliberately double. It can mean the sleep and slowness of age, and it can also carry the older sense of "rest" as the final rest of death. Because the rest happens "day and night," the sun-believer and the moon-believer are both covered. Neither belief gets the last word, and the poem's two heavenly bodies end up sharing a single phrase.
The form supports all this. There are no capitals and no punctuation, so the poem seems to drift rather than declare. The lines are short, running from three to six syllables, and the quiet sound-patterns do the work rhyme might otherwise do. There is no true rhyme, but "young" and "sun" echo each other as a near-rhyme, tying the first stanza's absent youth to the second stanza's bright daytime belief. The final line is made almost entirely of one-syllable words, "rest day and night," and the stresses fall slowly and heavily, so the poem seems to lie down on its last words.
What makes the poem worth reading is how much it holds without raising its voice. It shows that a short poem can contain a whole cosmology, a relationship and an attitude toward ageing. The attitude isn't denial or lament, but a dry, affectionate acceptance. The young may think of their beliefs as banners. By the end, "moon" and "sun" are only two names for the same long turning, and the poem finds a kind of comfort in that, with no argument at all.