The poem sets two kinds of light against each other. The sun, it says, is the "lord" of common people: it hangs high and distant, "quiet as a well," something everyone lives under but nobody can talk to. The title, "Lunatic," points toward the moon, and the second stanza seems to turn that way. The speaker imagines writing a letter to "a better lower one," a lord who is closer and, judging by the "her," female. The speaker even wants to hire her, month by month, which fits the moon's cycle.
The tone is quiet, playful and a little odd. The speaker seems to feel that the sun's authority is remote and unanswerable, and longs for a presence that can be addressed and bargained with. Treating the moon as someone to employ turns the old idea of a ruling power upside down: the speaker, not the lord, holds the contract. The mood is dreamy and whimsical, with a hint of loneliness underneath.