The point Frost is making in "On Looking Up By Chance at the Constellations" is that changes in the night sky happen very infrequently, but changes on Earth are not much more frequent.
He establishes that "a long, long time" will pass between events involving the planets, moon, sun, and stars. Because it happens so rarely, Frost suggests we can "look elsewhere than to stars and moon and sun For the shocks and changes we need to keep us sane."
However, the changes that happen on Earth are not predictable or easily observable, either. It is certain that every drought will eventually be ended by rain; he considers it a sure thing that peace in China will inevitably be ended by "strife," but these events are not any easier to isolate and observe than the changes in the heavens. Frost frequently refers to nature and uses parallels in the natural world to comment on people and their activities. This is another example.