A poignant moment occurs near the end of the first episode of “Cosmos: A Spacetime Odyssey,” a rollicking 13-part tour of the universe to be broadcast on Fox starting on Sunday.

Sitting on a rock by the Pacific, Neil deGrasse Tyson, host of the show and director of the Hayden Planetarium in New York City, pulls out an old desk calendar that had belonged to Carl Sagan, the Cornell astronomer and author. On a date in 1975 he finds his own name. The most famous astronomer in the land had invited young Neil, then a high school student in the Bronx with a passion for astronomy, to spend a day in Ithaca.

Dr. Sagan kindly offered to put him up for the night if his bus didn’t come. As Dr. Tyson told the story, he already knew he wanted to be an astronomer, but that day, he said, “I learned from Carl the kind of person I wanted to be.”

The story serves as a fitting handoff between the cosmic generations. The young student could hardly have guessed that he would grow up not only to be like Dr. Sagan but in some sense to be him.