The speaker is looking at a crescent moon with the rest of its disc faintly lit, a sight the poem describes with the old phrase "the old moon in the arms of the new." The dim part glows a "bleary orange" because of earthshine, sunlight bouncing off our planet and back onto the moon's dark face.
What stands out is the speaker's reaction. Instead of simple wonder, there is unease: "It doesn't seem right." The final lines suggest the speaker is troubled that we, meaning the Earth and perhaps the people on it, can give off "so much radiance." The title, "Wake," hints at a mood of mourning, as if the moon were being watched over, though the poem never says outright who or what is being grieved. Judging by the tone, the speaker may feel that something so bright shouldn't come from a world in trouble, or from loss.
The poem is quiet and short, and it leaves a mix of beauty, doubt and sadness hanging in the air.