Cosmos Book Summary - Cosmos Book explained in key points
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Cosmos summary

Carl Sagan

One small step toward understanding the greatness of the universe

4.4 (272 ratings)
21 mins

Brief summary

Cosmos by Carl Sagan takes readers on a journey through the universe, exploring the vastness of space and the mysteries of our existence. It delves into topics such as the origins of life, the evolution of intelligence, and the possibility of extraterrestrial life.

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    Cosmos
    Summary of 7 key ideas

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    Key idea 1 of 7

    Earth is truly tiny.

    The history of humankind has long been confined to earth. To us, it is everything, quite literally our world. But compared to the universe as a whole, the earth is really just a speck within a speck of dust. That’s because the size of the universe, or the Cosmos, is almost beyond comprehension.

    In fact, it’s so big that we’ve had to create a special unit of measurement based on the speed of light.

    Light is the fastest thing in the universe: in just one second it travels 186,000 miles or 300,000 km. That, in relatable terms, is equivalent to seven times around the earth.

    Based on that, when scientists talk about the Cosmos, they use light-years. That’s the distance that light travels in a whole year. To put a figure on it, about 6 trillion miles, or 10 trillion km!

    If that wasn’t already remarkable enough, consider that the Cosmos has contained within it roughly a hundred billion, or 1011, galaxies. And within each galaxy, there are roughly 1011 stars and 1011 planets.

    If you do the math, you’ll realize that our planet is one of 1022 planets in the Cosmos. Terrifyingly insignificant.

    Earth’s basic physical properties have long been known to humans. Around 2,000 years ago, scientists were already investigating its nature. They even calculated that the earth’s landmass was neither infinite nor flat.

    In the third century BCE, Eratosthenes, the director of the famous great Library of Alexandria in Egypt, worked out that the earth was a sphere.

    While reading a papyrus scroll one day, Eratosthenes learned that in Syene, modern Aswan, near the Nile, sticks cast no shadow at midday. This implied that at noon in Syene the sun was directly overhead.

    So Eratosthenes experimented. He placed a stick in the ground in Alexandria and observed that at midday there was a shadow in the city.

    From this, he concluded that the earth could not be flat. It had to be curved. If the land was flat, either both sticks would simultaneously have no shadow, or they would be at the same angle to the sun and therefore would have the same length of shadow.

    He even managed to use the difference in shadow lengths to calculate the circumference of the earth correctly. But he had to hire a man to pace out the distance between Alexandria and Syene (a walk of around 1,000 km) to get the final measurement he needed for the sum!

    This discovery was critical. Based on this knowledge, ambitious explorers set sail on little boats. How far they got, we may never know. But the spirit of exploration is still spurred on by science to this day. What are satellites but ships sailing through space?

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    What is Cosmos about?

    Cosmos (1980) is a milestone in popular science. It shows us the basic concepts behind our understanding of the universe, what the planets and the stars look like and how our comprehension of them has changed and evolved.

    Cosmos Review

    Cosmos (1980) by Carl Sagan is a captivating exploration of the universe, making it a must-read for anyone interested in our place in the cosmos. Here's why this book is worth your time:

    • With its fascinating insights about space and beyond, it opens our minds to the wonders of the universe, making us question our own existence.
    • Sagan's ability to distill complex scientific concepts into accessible language makes the book both educational and enjoyable for readers of all backgrounds.
    • Through Sagan's eloquent storytelling, Cosmos ignites a sense of wonder and curiosity that keeps us engaged throughout, proving that learning about the universe is far from boring.

    Best quote from Cosmos

    The surface of the earth is the shore of the cosmic ocean.

    —Carl Sagan
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    Who should read Cosmos?

    • Humanities students unsure of what gets scientists going
    • Lovers of science fiction
    • Anyone who’s ever looked up at the sky

    About the Author

    Carl Sagan was an American astronomer, author and famous popularizer of science. He co-wrote and narrated the television series Cosmos: A Personal Voyage, which was based on his best-selling book. It won him several awards, including an Emmy for Outstanding Individual Achievement.

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    Cosmos FAQs 

    What is the main message of Cosmos?

    The main message of Cosmos is the exploration of our universe and the awe-inspiring wonders it holds.

    How long does it take to read Cosmos?

    The reading time for Cosmos varies, but it typically takes several hours. The Blinkist summary can be read in just 15 minutes.

    Is Cosmos a good book? Is it worth reading?

    Cosmos is a captivating and thought-provoking book. It presents complex scientific concepts in an accessible manner, making it a worthwhile read for curious minds.

    Who is the author of Cosmos?

    The author of Cosmos is Carl Sagan.

    What to read after Cosmos?

    If you're wondering what to read next after Cosmos, here are some recommendations we suggest:
    • On the Origin of Species by Charles Darwin
    • Effective Decision-Making by Edoardo Binda Zane
    • A Brief History of Time by Stephen Hawking
    • The Particle at the End of the Universe by Sean Carroll
    • Who We Are and How We Got Here by David Reich
    • Brief Answers to the Big Questions by Stephen Hawking
    • Homo Deus by Yuval Noah Harari
    • The Leader In You by Dale Carnegie
    • Beyond the Pleasure Principle by Sigmund Freud
    • Beyond Good and Evil by Friedrich Nietzsche