The key ideas

Cosmos

Carl Sagan, 1980

More than two thousand years ago, a man measured the Earth with a stick and a shadow. Carl Sagan tells the history of science as a drama, where curiosity argues with superstition and testing argues with the wish to believe. From Kepler and the canals of Mars to the stars where the atoms of our bodies were born, this is a book about how the universe, through us, begins to understand itself.

Who it is for

For anyone who loves looking up at the night sky and wants to understand how people, step by step, found out where they live and what they are made of.

What counts is not what sounds plausible, not what we would like to believe, not what one or two witnesses claim, but only what is supported by hard evidence rigorously and skeptically examined.

What the book is about

More than two thousand years ago, Egypt had the Library of Alexandria. One day its head librarian read an odd note in a papyrus. In the southern city of Syene, at noon on the longest day of the year, the sun shone straight down to the bottom of a well, and a stick pushed into the ground cast no shadow. A detail like that is easy to walk past. He didn't, and on the next page you will find out what he made of it.

Carl Sagan, an astronomer and one of the best-known voices of popular science, published Cosmos in 1980 together with the television series of the same name. The book rests on one thought: science is the way people find out who they are and where they come from. Sagan tells the story of knowledge as a drama, where curiosity argues with superstition and testing argues with the wish to believe. Ahead are a stick and a shadow, an astronomer who trusted the numbers more than his own dream, canals on Mars that were never there, and the question of how our own story might end.

Idea 1. A stick, a shadow and the size of the Earth

The librarian's name was Eratosthenes. He knew that on the same day, at the same hour, a stick in Alexandria, north of Syene, did cast a shadow. How could that be? If the Earth were flat and the Sun very far away, the shadows in both cities would be the same. The simplest explanation for the difference is that the surface of the Earth is curved.

The length of the shadow gave an angle of about seven degrees, roughly one fiftieth of a full circle. The two cities were about eight hundred kilometres apart. Eratosthenes multiplied and found that the Earth was about forty thousand kilometres around, very close to what we know today.

As Sagan puts it, Eratosthenes had only sticks, eyes, feet and brains, plus a taste for experiment.

A Livria example: two identical cups of coffee cool at different speeds, and instead of shrugging it off, you ask yourself what is different about the places where they stand.

A big discovery sometimes starts simply: someone refuses to walk past a small mismatch and follows it all the way to a measurement. You don't always need an expensive instrument for that.

Read in Livria

About the author

Carl Sagan was an American astronomer and planetary scientist, a professor at Cornell University and a member of the teams that explored the planets. With Ann Druyan and Steven Soter he wrote the television series Cosmos, which came out in 1980 alongside this book.

All books by this author: Carl Sagan

Terms from the book

  • DNA
  • Kepler
  • Supernova
  • Viking
  • Papyrus
  • Library of Alexandria: The largest collection of manuscripts in the ancient world, in Alexandria, Egypt. Historians still debate exactly when and how the library was lost.
  • Syene: An ancient city in the south of Egypt, on the Nile. Today the city of Aswan stands in its place.
  • Eratosthenes: A Greek scholar of the third century BC and head of the Library of Alexandria. He worked out the circumference of the Earth from the length of a shadow.
  • Regular polyhedra: Solid shapes whose faces are all identical regular polygons, like a cube. There are only five of them.
  • Tycho Brahe: A Danish astronomer of the sixteenth century. Without a telescope, he made the most accurate observations of the planets of his time.
  • Ellipse: A stretched, closed curve. Kepler showed that the planets move around the Sun in ellipses, not in circles.
  • Lowell: Percival Lowell, a wealthy American businessman and astronomer who built an observatory in Arizona. He was sure he could see canals on Mars built by Martians.
  • Star-stuff: Sagan's image: the chemical elements of our bodies, apart from the very lightest, were once made inside stars.
  • Astrophysics: The branch of science that studies the physics of stars, planets, galaxies and the universe as a whole.
  • Drake equation: A formula by the astronomer Frank Drake that breaks the question of how many civilizations exist in the Galaxy into separate factors. It is a way of reasoning; many of the factors are still unknown.
  • Hypatia: A mathematician, astronomer and philosopher from Alexandria, one of the first women scientists we know of. She was killed by a mob in 415.

All terms

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