Cosmos Summary - Carl Sagan

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Cosmos Summary

This microbook is a summary/original review based on the book: Cosmos

Available for: Read online, read in our mobile apps for iPhone/Android and send in PDF/EPUB/MOBI to Amazon Kindle.

ISBN: 9780345539434

Also available in audiobook


Described by Dave Itzkoff of The New York Times as "a watershed moment for science-themed television programming," Carl Sagan’s 13-part TV-series “Cosmos” ran on PBS in 1980, and it inspired thousands of wide-eyed children into science and a life-long love of astronomy.

Same-titled, the book “Cosmos” is, more or less, a printed version of the series, with which it was originally co-developed. So, hop on Sagan’s “spaceship of the imagination” and get ready to explore the known universe, “all that is or ever was or ever will be”!

Eratosthenes vs. Flat-Earthers, 3rd century BC

In the third century B.C., in Egyptian Alexandria, there lived a man named Eratosthenes. An astronomer, historian, geographer, philosopher, poet, music theorist, theater critic, and mathematician, Eratosthenes was one of the foremost intellectuals of his day and age. Unsurprisingly for a man of such learning, he was also the chief librarian at the Great Library of Alexandria, the most significant library of the ancient world.

And it was there that, one day, Eratosthenes read in a papyrus book that in the southern frontier outpost of Syene – a town near the first cataract of the Nile – at noon on June 21, vertical sticks cast no shadows. This could mean only one thing: at noon, on the summer solstice, in Syene, the sun was positioned directly overhead.

How is it possible then – Eratosthenes asked himself – that at the very same moment a stick in Alexandria could cast a pronounced shadow? He had seen it before. "If Earth was flat," he thought, "then the sticks at both places should simultaneously cast no shadow" – or at least they would have the same shadow length if a flat Earth position had an angle.

Since neither of these things were true, the only possible answer was that Earth’s surface was not flat – but curved. Moreover, Eratosthenes mused – “since the greater the curvature, the greater the difference in shadow lengths should be” – it was theoretically possible to measure the Earth’s circumference. So, he did just that!

He first measured the angle formed by the stick and the sun’s rays at Alexandria by using the stick’s shadow as the third side of an imagined triangle. It turned out to be 7 degrees, which is something like one-fiftieth of the circumference of a circle. Meaning: Alexandria and Syene had to be about seven degrees along the Earth’s surface. 

Now all he needed was to measure the distance between the two cities. So, Eratosthenes hired men (known as “bematists”) to pace it out. They came back with 5,000 stadia,about 500 miles (800 km). Eratosthenes multiplied this number by 50 and got 25,000 miles (or 40,000 km) – which is almost the number modern science arrived at, thousands of years after.

The tiny Earth and the enormous Universe

Even though it probably seemed like a big number to the ancient Greeks, to modern people, 25,000 miles doesn’t seem that much, does it? After all, planes travel at an average speed of 600 mph (1000 kph), meaning a commercial passenger aircraft could fly around the Earth’s circumference in no more than two days. And yet, no plane has ever been invented that can leave the galaxy that contains our solar system, the Milky Way.

Miles and kilometers are impractical to describe our galaxy’s vastness, but in case you were wondering, its diameter should be about 6.2 x1017 miles (1x1018 km). We could translate that into words and say that the diameter of the Milky Way is in the range of quintillions, but would that mean anything to anyone?

Because the size of the cosmos is beyond ordinary human understanding, astronomers use different units of distance. When they talk about our solar system, they measure things in astronomical units. An astronomical unit (AU) is the average distance of the Earth from the sun, which means that 1 AU equals 150 million km. Mercury, for example, is about one-third of an AU from the sun,  and the distance between the sun and Jupiter is about 5 AU; Pluto, one of the furthest dwarf planets in the solar system is sometimes about 50 AU from the sun. This would put Pluto at about 7.5 billion km from the center of the solar system.

Unfortunately, even the astronomical unit is of no help when we talk about distances to objects outside our solar system. In this case, astronomers use a slightly eccentric unit of distance, since it sounds more as if a unit of time: the light-year. One light-year is the distance that light can travel in one year in a vacuum, which amounts to about 10 trillion km (6 trillion miles). To get some perspective of the vastness of our universe, consider these two facts: the sun is just eight light-minutes away from Earth, while Andromeda, the nearest galaxy to the Milky Way, is approximately 2.5 million light-years from our planet. 

Yes, that means precisely what you think it means: even if you could build a rocket that would travel at the speed of light (and it is theoretically impossible for anything to travel faster), you would still need 2.5 million years to reach Andromeda!

Travels in space and time

Now, as you know, you can only see objects if light rays reflect off them and enter your eyes; that’s why, for example, we can’t see things in the dark. But we just said that it takes eight minutes for the light emitted from the sun to reach Earth. Shouldn’t this mean that we can only see the sun as it was eight minutes ago?

As strange as it might sound, the answer is “yes”: the longer the light takes to reach us after being reflected off an object, the further back into the past of that object we can see. It sounds as if a premise for a science fiction  movie, but it is a basic fact of reality: whenever we look into space, we’re looking back in time. Even if you’re looking at a friend about three meters (10 feet) away from where you’re standing, you’re not seeing him as he is “now,” but rather as he “was” precisely a hundred millionth of a second ago (you get that number by dividing the distance between you two by the speed of light). 

But the difference between how your friend looks “now” and how he looked like a hundred-millionth of a second ago is too small to notice. On the other hand, in the unlikely event that the sun blew itself up five minutes ago, you would not know it for another three minutes, because that’s how long it would take for the information to reach us!

It was this kind of thought experiment (or Gedankenexperiments in German) that led Einstein to the discovery of two fundamental laws of our existence, sort of commandments of nature. Unlike the 10 shared by Moses with the Hebrews, these two can’t be broken. And they go something like this:

Thou shalt not add thy speed to the speed of light. Or, more scientifically, light (reflected or emitted) from an object travels at the same velocity whether the object is moving or stationary.

Thou shalt not travel at or beyond the speed of light. Or: No material object may move faster than light.

Most sci-fi movies feature man-made objects capable of traveling at speeds much faster than that of light. However, at least in our universe, for things to be logically consistent, there must be a cosmic speed limit. And that limit is the speed of light. No matter how hard we try, we should never be able to reach it, because, in that case, our world would scramble to incoherence and a bunch of illogicalities.

The wandering stars and their inhabitants

For most of history, humans were self-aggrandizing to a flaw. But, then again, it was almost intuitive for us to believe that we are unique creatures, fashioned by all-powerful beings as the pinnacle of our creative forces, and set on an enormous planet that is placed at the center of the known universe. After all, these should be some of the most natural ideas in the world: at sight, Earth indeed seems steady and immobile, while the heavenly bodies appear to rise and set each day around it just so that it can exist.

So, it should surprise nobody that – in the absence of experiments, observations, and mathematical proofs – every ancient culture leaped to the so-called geocentric model of the universe, with Earth serving as its center, and the sun, the moon, and the planets orbiting around it. It would take millennia before a Polish Catholic clerk named Nicolaus Copernicus would come up with the radical theory that it is, in fact, the other way around: it is Earth and the other planets that rotate around the sun which, he believed, must be the center of the universe. 

This happened in 1543. From then on, the universe just kept getting bigger and bigger: we discovered that the sun isn’t the center of the universe, but the center of a relatively tiny solar system, which is, in fact, just one of tens of billions of other solar systems in our galaxy, the Milky Way. Up until just a century ago, most astronomers couldn’t fathom the size of the Milky Way and believed that it must contain all the stars in the universe. Now, we know that it’s just one of some hundred billion (1011) galaxies, each with, on average, a hundred billion stars and about as many planets. How is it possible then that we are the only conscious beings around? Where are all the aliens?

The answer is: we don’t really know. But if they do exist, Sagan warns, they are most probably not like us since they would have evolved in fairly different environments. “The Darwinian lesson is clear,” he writes lyrically, “there will be no humans elsewhere. Only here. Only on this small planet. We are a rare as well as an endangered species. Every one of us is, in the cosmic perspective, precious. If a human disagrees with you, let him live. In a hundred billion galaxies, you will not find another.”

The star-stuff that speaks for the stars

“To what purpose should I trouble myself in searching out the secrets of the stars,” Anaximenes supposedly asked Pythagoras in the sixth century before Christ, “when death and slavery are continually before my eyes?” We don’t know Pythagoras’ answer, but we do know that this question is still a valid one. After all, in a slightly different form, it is the one religion apologists frequently posit to scientists and new age atheists:  indeed, if there is only evolution and no god, is there a meaning to our being here?

Of course there is, says Sagan – and it is precisely our being here. After all, there are thousands and thousands of worlds in the vast universe on which life has never arisen. And probably thousands more on which it was destroyed at one point or another in history. We are the fortunate ones – for we have become, and we have survived. And we have sprung from the same stuff that has created the stars: you could argue that, in a way, they are our Zeuses and Jahwehs and Allahs. The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies – they were all made in the interiors of collapsing stars. We are literally made of star-stuff. The cosmos, as Sagan says, “is tangibly within us.” And because, unlike anything else we know, we are also conscious, we are not only the pinnacle of the cosmos’ evolution but also a way for the cosmos to know itself.

“We are the local embodiment of a Cosmos grown to self-awareness,” explicates further Sagan. “We have begun to contemplate our origins: star-stuff pondering the stars; organized assemblages of ten billion billion billion atoms considering the evolution of atoms; tracing the long journey by which, here at least, consciousness arose. Our loyalties are to the species and the planet. We speak for Earth. Our obligation to survive is owed not just to ourselves but also to that Cosmos, ancient and vast, from which we spring.”

Final Notes

Cleverly structured and imaginatively illustrated, Sagan’s “Cosmos” is arguably the best book on astronomy ever written. Lyrical and iridescent, it might also be one of the best nonfiction books in history as well.

“If we send just one book to grace the libraries of distant worlds,” wrote David Whitehouse for BBC in 2010, “let it be ‘Cosmos’.” 

We are not far off from seconding that.

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Who wrote the book?

Carl Sagan (1934-2006) is most known for his life-long efforts in popularizing science. Aside from the television series described above, he authored numerous books including “The Dragons of Eden” and “Pale Blue Dot,” spent decades... (Read more)