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 (214 ratings)
21 mins
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    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.

    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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