In this poem, a husband and wife are sitting in their cottage when the man asks the wife why she is staring at the candle so intently: "Unblinking with those great lost luminous eyes?" She replies that she is watching a moth burn in the flame. Fall is coming. The husband comments that "moths fly in from the heather" now that the days "decline." The wife tells her husband she is going out into the moonlight, and her husband, "little heeding," goes back to his history book. Outside, the woman meets her lover. She says the moth he sent as signal to her to meet him is all burnt up, and she comments she might be better off suffering the same fate, because she has broken her marriage vows. Then a ghostly "Ancient Briton" speaks from a "tumulus" or ancient burial ground, that is now covered with pine trees, to say that thwarted love has not changed between his day and the present.
In the first part of the poem, we sense the distance and tension between the husband and wife. They are in the same room but in different worlds, unable to communicate. He is lost in his history book: "the annal of ages gone." This is ironic because he misses the "age old" (as the "Ancient Briton" tells us) drama of adultery going on right beneath his nose. The wife, meantime, is also lost, staring at the moth in the flame and seeing herself in it.
The moth in the flame is a dark, unpleasant image. 'Its wings and legs are turning/To a cinder rapidly," the woman tells her husband, and later she says to her lover that it is "burnt and broken." The moth represents the woman, who feels trapped between an unsatisfying marriage and a love affair she believes is wrong because it has "shattered" her "marriage troth [vows]." We see how hopeless and unhappy she feels as she identifies with the "poor moth." She seems to blame her lover for her destructive affair which she believes is going to end up leaving her "burnt and broken." "You lured me out," she accuses him. Their passion is not the warming fire of love but the destructive fire that incinerates a helpless creature.
The poem, in which nobody has a name, and little is described in detail except for the moth and the burial mound, underscores the timelessness of this situation. We know nothing about the wife or the husband or the lover.
The last stanza, in which a ghost suddenly speaks, can be confusing. We might remember that, although a rationalist, Hardy liked to put ghosts into his poems, doing so more than 40 times. In "Disembodied Voices in Hardy's Shorter Poems,: Vern Lentz notes that "The Moth-Signal" ...[uses a] spectral voice to comment rudely and tauntingly on the romantic affairs of men and women. ... the ghost speaks only at the end ... the ghost's voice is a jarring intrusion into a dramatized human situation."