Carl Sagan (1934–1996) was an astronomer at Cornell University who worked on NASA's missions to the planets, including the Viking landers on Mars and the Voyager probes, for which he helped make the Golden Record carrying sounds and pictures from Earth. He was also one of the best-known science explainers of his time. His television series Cosmos: A Personal Voyage (1980) was watched by hundreds of millions of people, and the book Cosmos came out the same year.
Key idea
Borrow the book
Cosmos by Carl Sagan is still under copyright, so we do not reproduce it. Find it at a library: Open Library or WorldCat.
The Cosmic Calendar
Sagan's most famous teaching tool squeezes the whole history of the universe into one calendar year. The big bang is the first moment of January 1; now is midnight at the end of December 31. Sagan used the best age known at the time; we use about 13.8 billion years.
Worked example: Calendar math
- $13{,}800{,}000{,}000 \div 365 \approx 37{,}800{,}000$.
Answer: About 38 million years per day.
Worked example: The dinosaurs
- One day is about 37.8 million years.
- $66 \div 37.8 \approx 1.75$ days.
Answer: About 1.75 days: on December 30.
Reading guide, chapter by chapter
| Chapter | What it is about, in our words |
|---|---|
| 1. The Shores of the Cosmic Ocean | A tour of the universe from galaxies to Earth, and Eratosthenes measuring the Earth with shadows and a stick, around 240 BC. |
| 2. One Voice in the Cosmic Fugue | Evolution and the chemistry of life; how all living things are related. |
| 3. The Harmony of Worlds | Astrology versus astronomy, and Johannes Kepler's long struggle to find the true shapes of planetary orbits. |
| 4. Heaven and Hell | Comets, impacts and the scorching greenhouse of Venus. |
| 5. Blues for a Red Planet | Mars in story and in science, and what the Viking landers found. |
| 6. Travelers' Tales | The Voyager probes and the age of Dutch explorers and scientists like Christiaan Huygens. |
| 7. The Backbone of Night | What the stars are, and how the early Greek thinkers of Ionia began asking natural questions. |
| 8. Travels in Space and Time | Light speed, relativity, and what travel to the stars would mean. |
| 9. The Lives of the Stars | How stars are born, shine and die, and how they made the atoms in us. |
| 10. The Edge of Forever | Galaxies, the expanding universe and the big bang. |
| 11. The Persistence of Memory | Genes, brains, books and how information is stored. |
| 12. Encyclopaedia Galactica | The search for life beyond Earth and how we might talk to it. |
| 13. Who Speaks for Earth? | The Library of Alexandria, the cost of losing knowledge, and humanity's future. |
Common mistake
| When | What happened |
|---|---|
| 1934 | Sagan born in Brooklyn, New York |
| 1977 | The Dragons of Eden, where the Cosmic Calendar first appears in a book |
| 1977 | Voyager probes launched with the Golden Record |
| 1980 | Cosmos, the series and the book |
| 1990 | Voyager 1 takes the Pale Blue Dot picture of Earth |
| 1996 | Sagan dies in Seattle |
Read more: Carl Sagan, Cosmos (the book) and Cosmos: A Personal Voyage.