Without the narrator's speculation, there would be no resolution to the story at all. Instead, we would simply be left with the anomaly of the little pyramid, the sentinel, obviously the product of a greatly advanced technology, standing upon the moon - a world which shows no other evidence at all of a native civilization, let alone one so developed as the one which must have constructed the pyramid. The narrator, Wilson, does his best to fill in the background to this puzzle through speculation. The fact that he is obviously an intelligent and erudite man, and a frequent visitor to the Moon - 'selenologist', as he describes himself - lends a touch of credibility to his musings.
Wilson manages to move beyond the more obvious, initial assumption that the sentinel provides evidence of an otherwise long-vanished native moon civilization, to a truly cosmic vision. He arrives at the intriguing conclusion that the sentinel has been left there by a truly ancient and all-powerful race, one of the first great civilizations of the universe, who passed through our solar system a long, long time ago, perhaps before humans even existed. However, Wilson thinks that now that mankind has discovered the sentinel, it has in effect alerted that ancient race to the fact that an intelligent civilisation now exists on earth, and so they will return. He puts it thus:
If you will pardon so commonplace a simile, we have set off the fire alarm and now have nothing to do but wait.
I do not think we will have to wait for long.
The story ends on this tantalizing note, with the sense of waiting for a momentous event, the visit from this ancient civilization to earth. It is an open ending to the story, fuelling a sense of suspense. Wilson increases this suspense even further by pointing out that there is no way of knowing whether this civilization will be benevolent or not. In fact, he tends to the latter opinion:
But they must be very, very old, and the old are often insanely jealous of the young.
In this way the story sounds quite an ominous note as to the potential future of the human race.
Wilson's speculations, then, serve a very valuable purpose in the story, filling in a credible background to the odd discovery of the sentinel which makes up the main narrative. He provides a resolution in the sense of providing an explanation, yet at the same time he leaves room for further speculation and an ongoing sense of tension. His musings also heighten the poetic feel of the story with his cosmic imaginings, for example with the strikingly beautiful and apposite image of our solar system in its early stages, when, he says, the planets 'were warming themselves around the inner fire of the Sun and waiting for their stories to begin'. Earth's story, Wilson suggests, has now reached a crucial stage.