Carl Sagan

Early in Cosmos, Carl Sagan takes us across an enormous universe, then brings us home to Earth. Here, the journey becomes something wonderfully human: the story of a person who wanted to know how big his world was.

His name was Eratosthenes. He lived in ancient Alexandria, more than two thousand years before anyone could photograph Earth from space.

Sagan explains how sunlight and a shadow could help him measure a planet. Near Syene, the summer-solstice noon Sun was almost directly overhead. In Alexandria, farther north, objects still cast shadows. Using the angle of a shadow, the distance between the cities, and the geometry of a curved Earth, Eratosthenes estimated the distance around the world.

A whole planet, measured from the ground.

Watch Sagan tell it. His delight makes an ancient calculation feel like an event worth celebrating. He takes time to explain the reasoning, so that we can follow Eratosthenes into the discovery. Across all those centuries, another person’s mind becomes a little less distant.

Later in Cosmos, Sagan introduces Johannes Kepler, who searched for order in the movements of the planets. Kepler hoped to find perfect circles. The observations collected by Tycho Brahe led him to ellipses instead: the stretched circles along which planets travel around the Sun.

Sagan gives this discovery a human life around it. His Kepler is a person with cherished ideas, disappointments, and a longing to understand the sky. We come to appreciate the individual who did the work, as well as the knowledge he left behind.

That is part of the generosity of Cosmos. Sagan makes room in our imaginations for people we never had the chance to meet. Their names become attached to something we can recognize: the excitement of a question, the care of an observation, the astonishing reach of a human idea.

There is something deeply thrilling about a person looking at the world and figuring something out. A shadow becomes a way to measure a planet. An understanding of nature becomes a way to grow food, cure disease, or build a city. Through reason and imagination, we turn what we discover into new possibilities for living. Sagan makes that human power feel magnificent. Watching these individuals reach for understanding, we recognize something fundamental about ourselves: we flourish by using our minds to discover and create.

In an essay introducing Cosmos, he explained that the series should reach our hearts as well as our minds. He would be pleased, he wrote, even if viewers forgot its facts, provided it renewed their joy in understanding nature.

He could make that joy feel very close. The carbon in our bodies was formed inside stars. Consider what that means when a child raises a hand to point at the night sky: the hand and the stars belong to one unfolding history.

Seen this way, the people who help us understand the universe become precious. Their discoveries enlarge the world we can experience. Eratosthenes gives us a way to grasp the size of Earth. Kepler helps us recognize order in the planets’ journeys. Sagan helps us feel wonder at both—and affection for the people who made them possible.

Sagan, too, belongs in that company. His gift was to bring the discoveries home, until we could look up at the sky with a deeper understanding of what we were seeing and whom we had to thank.