The poem sets a surprising fact against a gloomy day. Earth is at perihelion, its nearest point to the sun, yet the air is damp and the sky hums gray. The speaker finds it hard to believe, and that disbelief carries the poem's mood: nearness in space doesn't bring warmth or connection.
The pantoum form does a lot of the work. Lines return and repeat, shifting slightly each time, so the poem feels like it's circling, much like the planet's orbit and the speaker's own restless looking. The speaker turns to find "live things" but sees only doves that won't sing their mourning, and a stranger at a distance. Even the word "perch" shifts from the doves to the speaker, suggesting both are alone.
The feeling is quiet loneliness, a sense of being stuck indoors or apart. The final line returns to the opening fact, leaving the contrast unresolved: close to the sun, yet distant from other people.