The poem imagines a world where night no longer exists, and the title suggests why: we are living through an age of extinction. Read that way, "no night" could mean the loss of real darkness, perhaps through constant artificial light, or it could stand for a larger absence, a world emptied of the things that once gave it depth and mystery. The poem never says which, and that openness is part of its power.
The second stanza shifts to a quiet, almost stunned observation: "how small / the others seem." Without darkness, other things, whether stars, creatures, or other lives, shrink in importance. The last stanza lands hard. We have "fallen again / to arithmetic," as if we keep retreating into counting and measuring, reducing living things to numbers and losses to statistics. The word "again" hints that this is a habit, not a one-time slip.
The speaker sounds subdued and quietly grieving. The short lines and wide gaps create a hushed, stark mood, like someone speaking softly after a loss.