Penny drives Gordon to her parents' house, but Gordon's mind is elsewhere, trying to come at this problem from a different angle.
Gordon gets along well enough with Penny's dad, who even has a layman's love of astronomy, but throughout the whole vacation, Gordon's mind keeps wandering back to the problem and looking at it from a different angle.
Borrowing Jack's books, Gordon finds nothing about Hercules.
On the way home, they stop at the Berkeley campus and Gordon heads straight for the physics department library; poring over books, he notices a passage about solar motion.
He quickly does some calculations.
On the flight back to San Diego, Gordon tells Penny about his calculations. He learned that the sun—and the earth with it—someday will be located where Hercules is today.
But he has no idea what that means in terms of the message.