The poem sets up a famous measurement, the distance to the moon, only to knock it down with a quiet comparison. The speaker states the figure, 384,400 kilometres, as if it were the biggest gap imaginable. Then comes "but," and the whole thing tips: somehow, the person they're addressing feels even farther away.
What makes it work is the layout. The words drop one per line, so the poem reads slowly, almost like a pause between breaths. The final lines, "a / little / more / distant," stretch out the admission and give it a hesitant, deflated feel. "Somehow" matters too. The speaker can't explain how someone close could seem farther than the moon, and that confusion is part of the hurt.
The mood is lonely and understated. There's a dry, wry edge in comparing a heartbreak to astronomy, but underneath it the poem suggests emotional distance can feel more impossible to cross than physical space, because no rocket can close it.