Science
by Carl Sagan
How did we find our place?
Carl Sagan wrote Cosmos alongside his 1980 TV series. It asks how a species on one small planet worked out where it sits in an enormous universe, and what it might do with that knowledge. He tells it through the people who found things out.
Hear a sample · read by Laura
Sagan explains that the carbon and iron in your body were made inside stars. Older stars built these elements and scattered them as they died, some in huge explosions. They ended up in the cloud of gas and dust that collapsed to form the Sun and its planets.
Johannes Kepler cast horoscopes to earn money, but his real work was on the orbits of the planets. Tycho Brahe's precise measurements of Mars wouldn't fit a circle, so Kepler gave up the perfect circles he loved and found that orbits are ellipses. Sagan admires him for letting evidence win.
Venus is covered in thick cloud, and people once imagined swamps underneath. Measurements showed a surface hot enough to melt lead, heated by a runaway greenhouse effect in its carbon dioxide air. Sagan, who studied Venus, saw it as a warning to Earth, where we keep adding carbon dioxide.
Cosmos ends on a worry about our survival. Sagan tells of Hypatia, a scholar in Alexandria killed by a mob as the ancient world's learning was being lost. He sets that beside the nuclear arms race of his day, and asks who will stand up for the whole planet.
The shore
Earth is a tiny world in a huge cosmos, and we've only begun to explore it.
One family
All life on Earth uses the same chemistry and belongs to one family tree.
Kepler
Kepler dropped a theory he loved when careful measurements showed it was wrong.
Star stuff
The elements in our bodies were cooked inside stars long ago.
Alexandria
When the Library of Alexandria was lost, much of the ancient world's learning went with it.
A story from the book
Eratosthenes, a librarian in ancient Alexandria, read that at noon on the summer solstice, sticks in Syene cast no shadow. In Alexandria they did. He realised the Earth must be curved. From the shadow's angle and the distance between the cities, he worked out its size, and came remarkably close.
Hot take · true or myth?
People have reshaped animals and plants by choosing which ones breed.
True. Sagan calls this artificial selection. Farmers and breeders reshaped animals and crops over thousands of years. Natural selection, he points out, has had billions of years to work.
One thing to try
Look for Venus this week. When it's visible, it's the brightest thing in the night sky after the Moon, low in the west after sunset or in the east before dawn. A free sky app shows where to look. While you look, remember what its clouds are hiding.
Knotd has the key ideas from 250+ non-fiction books, each in a few minutes, in plain words. Cosmos comes with a Hot Take round to make it stick.
Get the whole book on KnotdOpen in the appHow did we find our place? Carl Sagan wrote Cosmos alongside his 1980 TV series. It asks how a species on one small planet worked out where it sits in an enormous universe, and what it might do with that knowledge. He tells it through the people who found things out.
Made inside stars; Kepler lets go of circles; What happened to Venus; Who speaks for Earth?.
Look for Venus this week. When it's visible, it's the brightest thing in the night sky after the Moon, low in the west after sunset or in the east before dawn. A free sky app shows where to look. While you look, remember what its clouds are hiding.
Yes. Knotd has the key ideas from Cosmos in a few minutes, in plain words, plus a Hot Take round to test them. It's one of 250+ non-fiction books in the app.
