Cosmos Book Summary, by Carl Sagan

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1-Page Summary of Cosmos


Sometimes life can be difficult and overwhelming. Sometimes it feels as if your entire world is falling apart, but there are billions of people in the world, and the earth itself isn’t that important in comparison to everything else out there.

Carl Sagan was a great communicator. He made difficult things relatable to the masses, and that’s why he was so successful. A lot of people don’t understand science, but it doesn’t have to be complicated. It can be fun and interesting if you know what you’re talking about and are able to present it in an engaging way. That’s why I’ve collected some key points from Carl Sagan’s lectures on space exploration history that will allow others to learn about our place in the universe without getting bogged down by boring math or complex scientific concepts.

This book contains a whole bunch of interesting facts about aliens, Greek philosophers and Einstein’s thought experiments.

Big Idea #1: Earth is truly tiny.

Humankind has long been confined to the earth. Our entire history is on this planet, and we consider it everything because of its size. Compared to the universe as a whole, though, our world is just a speck of dust within another speck of dust.

That’s so big that we had to create a special unit of measurement for it.

Light is the fastest thing in the universe. It travels 186,000 miles per second or 300,000 kilometers per second, which means that it can go around the earth seven times in a single second.

Because of that, scientists measure the distance between stars in light years. That’s how far light travels during one year. It’s about 6 trillion miles or 10 trillion kilometers!

The universe has a very large number of galaxies. Each galaxy contains billions or trillions of stars, and each star may have planets in orbit around it.

If you do the math, you’ll realize that our planet is one of 1022 planets in the universe. That’s a lot of planets for us to be so insignificant.

Humans have known about the earth’s basic physical properties for a long time. Around 2,000 years ago, scientists were already investigating its nature and learning that it was not flat or infinite. In the third century BCE, Eratosthenes worked out that the earth was a sphere by observing shadows in Syene while reading a papyrus scroll one day.

Eratosthenes experimented with the sun and sticks. He observed that at midday there was a shadow in Alexandria, so he concluded that the earth is curved. If it were flat, they would have had equal shadows or no shadows at all, but they didn’t.

Eratosthenes was able to use the difference in shadows at two different points on earth to calculate how big it is. He had a man walk between those two places and used that measurement to find out the size of the planet. With this information, explorers began sailing across oceans. Even today, satellites help us explore our universe and learn more about planets outside of our solar system.

Big Idea #2: The stars and planets have always called to us, teaching us about the earth and its place in the universe.

Humans have tried to understand the stars for thousands of years.

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Early humans used the stars to keep track of time and date important events. They also realized that they could use them to navigate.

The ancient Egyptians also used the stars to determine when certain fruits would be ready for harvest, as well as when animals were migrating.

The movement of heavenly bodies is predictable. If you track their movements over time, they’re constantly looping around the sky.

This idea was proposed by Ptolemy, who worked in the Library of Alexandria during the second century CE. He thought that the sun and stars revolved around the earth.

Cosmos Book Summary, by Carl Sagan

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