Book SummaryCosmos, by Carl Sagan

1-Page Summary1-Page Book Summary of Cosmos

Cosmos is astronomer Carl Sagan’s exploration of the universe. In this book, Sagan gives a comprehensive description of the science, philosophy, and history of astronomical discovery, from ancient Ionia to the time of the book’s publication in 1980. He discusses some of the obstacles posed by religious belief and institutions to scientific inquiry throughout history, and he celebrates the victories of the scientific endeavor. Although much research has been done in astronomy since the publication of Cosmos, the book remains timeless in the sense of wonder and awe it evokes. Sagan provides us with food for thought on some of the biggest questions pondered by humanity—questions about our place and significance in the vast universe.

Sagan was an American astronomer who spent most of his prolific career in [Cornell...

Cosmos Summary Part 1: The Vastness of Space and Time

In this section, we’ll discuss Sagan’s description of the vastness of the cosmos (or universe), and our place in it. We’ll begin by examining how life evolved on Earth to find our place within that timeline. After situating ourselves, we’ll zoom out to look at Sagan’s bigger picture of where Earth fits into the timeline and space of the universe. Ultimately, when we look at what’s known about the cosmos, we’ll discover that in the grand scheme of things, we’re pretty insignificant.

Humanity’s Place on Earth

Sagan begins the story of humanity’s history by teaching us that all life is made of carbon-based organic molecules, but there was once a time on Earth without life. How was it possible for life to come from non-life? Let’s look at a timeline of how that process occurred and discuss Sagan’s thoughts on whether the same thing could occur on other planets.

About 4.6 billion years ago, the Earth formed out of condensed gas and dust. The fossil record tells us life arose about 4 billion years ago, meaning that for 600 million years there was no life on Earth. According to Sagan, hydrogen-based molecules that are found throughout space were gradually broken...

Cosmos Summary Part 2: History of Astronomical Thought

Humans in all places and times have been fascinated by the skies. We’ve used the stars to guide us, to mark the passage of seasons, and make sense of our place in the world for as long as we’ve existed. But our understanding of the nature and order of the universe, and our place in it, has evolved substantially over time. Sagan gives a historical picture of how humanity developed its beliefs “from chaos to cosmos.”

In this section, we’ll briefly outline the Western history of astronomical discovery, as presented by Sagan, from Thales of Miletus in ancient Ionia, to Isaac Newton in 17th-century England.

The Ionian Awakening

Sagan’s history begins between 600 and 400 BC, when a revolution of scientific thought occurred in Ionia, a Greek settlement on the Aegean coast in what is now Turkey. Ionia is often considered the birthplace of science, in the Western tradition. This revolution was sparked by a scientist named Thales of Miletus, who Sagan says was the first known scientist bold enough to attempt to explain the world without appealing to the supernatural. He explains that this had far-reaching consequences, as it prompted a shift from a world made by gods to...

Cosmos Summary Part 3: Exploration of Space

Now that we have a solid working model of the universe, and have been able to directly observe much of it via telescopes, we move into the age of physical exploration. We’ll begin with discussing what modern astronomers have discovered about the celestial bodies in our own solar system, then look at the potential that lies beyond our solar system. Sagan offers some insightful views on the potential for interstellar travel and for discovery of life on other worlds.

Our Solar System

On July 20th, 1969, humans made the first trip to a destination off the planet Earth and set foot on the moon. The moon is the nearest celestial body to us, and it remains the only one humans have ever traveled to. However, Sagan explains that we’ve been able to create vehicles that can travel much further, enduring the inhospitable climates elsewhere in the solar system, and send information to us.

At present, Sagan says that none of the atmospheres and climates of the other bodies in our own solar system are inhabitable by anything like the kind of life we have here on Earth. But he reminds us that life could take any number of forms, and because it would have evolved in a different...

Shortform Exercise: Contemplate Our Insignificance in the Cosmos

Sagan emphasizes how the vastness of the cosmos highlights our insignificance. Ponder your place in the universe.

Thinking about these questions can cause existential anxiety for some people. But it can also be liberating and empowering. How do you feel when you contemplate the vastness of the universe as compared to this “pale blue dot” you live on?