The poem is set in a classroom, but the speaker looks at the students and sees something larger. Calling them "seas of tranquility" is a quiet joke and a tender image at once. The phrase echoes the name of a region on the moon, so the room becomes a landscape: calm, distant, a little unreal. The students "sort of nod," a casual, half-asleep gesture, and the speaker reads it as a kind of silent message: "in a little while it won't exist."
What won't exist is left open. It could be this classroom, this moment, or the whole world of youth and learning. The final line, "not even on postcards," adds a wry, sad touch. Not even the small souvenirs people keep will hold on to it.
The mood is gentle, wistful, and slightly dreamy. The speaker doesn't panic about loss. He notices it lightly, almost offhandedly, which makes the feeling of time slipping away land more softly and stay longer.