Carl Sagan was born in Brooklyn, New York. His father, Samuel Sagan, was an immigrant garment worker from Russia, in today's Ukraine. His mother, Rachel Molly Gruber, was a housewife from New York. Carl was named in honor of Rachel's biological mother, Chaiya Clara, in Sagan's words, "the mother she never knew". Sagan graduated from Rahway High School in Rahway, New Jersey, in 1951.
He had a sister, Carol, and the family lived in a modest apartment near the Atlantic Ocean, in Bensonhurst, a Brooklyn neighborhood. According to Sagan, they were Reform Jews, the most liberal of Judaism's three main groups. Both Sagan and his sister agree that their father was not especially religious, but that their mother "definitely believed in God, and was active in the temple ... and served only Kosher meat". During the depths of the Depression, his father had to accept a job as a theater usher.
According to biographer Keay Davidson, Sagan's "inner war" was a result of his close relationship with both of his parents, who were in many ways "opposites". Sagan traced his later analytical urges to his mother, a woman who had known "extreme poverty as a child" and had grown up almost homeless in New York City during World War I and the 1920s. As a young woman she had held her own intellectual ambitions, but they were frustrated by social restrictions: her poverty, her status as a woman and a wife, and her Jewish ethnicity. Davidson notes that she therefore "worshiped her only son, Carl. He would fulfill her unfulfilled dreams".
However, his "sense of wonder" came from his father, who was a "quiet and soft-hearted escapee from the Czar". In his free time he gave apples to the poor or helped soothe labor-management tensions within New York's "tumultuous" garment industry. Although he was "awed" by Carl's "brilliance, his boyish chatter about stars and dinosaurs", he took his son's inquisitiveness in stride and saw it as part of his growing up. In his later years as a writer and scientist, Sagan would often draw on his childhood memories to illustrate scientific points, as he did in his book, Shadows of Forgotten Ancestors. Sagan describes his parents' influence on his later thinking:
My parents were not scientists. They knew almost nothing about science. But in introducing me simultaneously to skepticism and to wonder, they taught me the two uneasily cohabiting modes of thought that are central to the scientific method.