Cosmos
Carl Sagan
Cosmos
Carl Sagan
An astronomer's journey through space and history: how stars, life and science came to be, and what our place in the universe asks of us
- No ratings yet
- 12 key ideas
- Text
- Published 1980
Introduction
Carl Sagan, the Cornell astronomer, takes the reader on a tour of the universe and of the people who learned to understand it. Over thirteen chapters he moves from the vastness of space and the Library of ancient Alexandria to the evolution of life, Kepler and Newton, the catastrophes that shape planets, the search for life on Mars, the Voyager probes, the Ionian Greeks who first sought natural causes, relativity and starships, the lives and deaths of stars, the Big Bang, the storage of knowledge in genes, brains and books, the search for other civilizations, and, at the end, the danger of nuclear war and the hope that humanity will act as one species.
The book was written together with the thirteen-part television series Cosmos, which Sagan made with Ann Druyan and Steven Soter, and each episode follows its chapter fairly closely. This summary follows the text of the 1980 book.
Notes: the science is as Sagan gave it in 1980, and some of it has moved on. He puts the age of the universe at ten to twenty billion years; measurements by the Planck satellite, published in 2013, put it at about 13.8 billion. He counts nine planets; in 2006 the International Astronomical Union classed Pluto as a dwarf planet. He hoped to learn within decades which nearby stars have planets; by 2025 NASA counted more than 6,000 confirmed planets around other stars. He expected the Voyagers to leave the Sun's realm in the middle of the twenty-first century; Voyager 1 crossed that boundary in 2012 and Voyager 2 in 2018. The shortage of neutrinos from the Sun, which he calls an unsolved mystery, was explained in 2001: the neutrinos change type on their way to Earth. He leaves open whether the universe will expand forever or fall back; in 1998 astronomers found that its expansion is speeding up. He writes that no one knows what killed the dinosaurs; in 2010 a panel of scientists concluded that an asteroid impact was the cause. What Sagan says about the Library of Alexandria, Hypatia and religion is his own account and view, reported as his, and this summary takes no position on it.
- 1Sagan argues that the universe is vast and mostly empty, that Earth is a rare and precious place in it, and that science lets us understand it
- 2Sagan explains that all life on Earth evolved by natural selection from one shared origin, and that its chemistry may be common in the universe while its forms are not
- 3Sagan shows how Kepler gave up a beloved idea to fit Tycho Brahe's measurements, found the laws of planetary motion and prepared the way for Newton
- 4Sagan warns that worlds can be wrecked, by impacts from space or by a runaway greenhouse like the one that made Venus a hell
- 5Sagan uses the story of Mars to show how strong wishes can fool careful observers, and why only evidence can settle whether there is life there
- 6Sagan presents the Voyager probes as heirs of the sailing ships of discovery, bringing back travelers' tales of strange new worlds
- 7Sagan argues that science was born among the Ionian Greeks, who looked for natural causes, and faded when mysticism and slavery won out
- 8Sagan explains that looking out into space means looking back in time, and that relativity makes the stars reachable in principle but very hard in practice
- 9Sagan shows that stars are born, live and die, and that the atoms of our bodies were made inside them
- 10Sagan describes how the universe began in a Big Bang ten to twenty billion years ago, and asks whether it will expand forever or fall back again
- 11Sagan traces how knowledge is stored in genes, brains and books, and argues that listening for messages from other civilizations is worth the effort
- 12Sagan argues that the technology that could take us to the stars could also destroy us, and that we must act as one species that speaks for Earth
This book is for Premium members
The introduction is free. Premium opens every key idea, in audio and text.