This is one of the few Bradbury short stories that has never been reprinted since its appearance in the Spring 1948 issue of PLANET STORIES.
It's about an old man named Nibley who has a unique, almost supernatural ability to calculate the orbits of objects in motion. That's a valuable ability to have for navigating in space, of course, but science has passed Nibley by and all rockets now have radar computators to handle such navigation. But then an accident happens and the fate of the entire Earth colony on Jupiter relies on getting a supply ship through the Asteroid Belt without its instruments, with only a dying old man to guide the way . . .