Carl Sagan was born in Brooklyn on 9 November 1934, which makes today his ninety-second birthday, and he has been dead for thirty years. He died in Seattle on 20 December 1996, at sixty-two, of pneumonia that followed a rare bone-marrow disease (Britannica, NASA).

What survives of him in common use is a manner: a calm voice explaining something enormous, and the impression that the cosmos is friendly to the curious. That is a comfortable memory, and it is not the reason his work holds up. The reason is harder to like and easier to test. Sagan spent his life performing corrections, most of them on subjects people had already decided they loved.

The sentence that rearranged him

He described himself as a childhood science fiction addict who became fascinated by astronomy when he learned that every star in the night sky was a distant Sun (NASA). Notice the shape of that fact. It is not an addition to the sky. It is a replacement for the sky everyone had been looking at, delivered in one sentence, available to any child who asks what a star is.

He took four degrees in physics, astronomy and astrophysics from the University of Chicago, then worked on planetary atmospheres at Berkeley, Harvard and the Smithsonian before Cornell made him director of its Laboratory for Planetary Studies in 1968 (Britannica). From that chair he helped explain three things that had resisted explanation, and all three are the same story told at different temperatures: what a planet’s air is doing to its surface.

Venus is hot because of a massive greenhouse effect. The seasonal changes on Mars are windblown dust. The reddish haze of Titan is complex organic molecules (The Planetary Society). Each answer replaced a more romantic one, and each required somebody to build an instrument, model the atmosphere, and accept the number that came back.

That is the discipline to which I keep returning. It is easy to be curious about a planet. It is much harder to be curious in a way that can lose. A greenhouse model of Venus can be wrong; a poem about Venus cannot. Sagan chose the kind of statement that could fail, over and over, on worlds where nobody was watching and no career depended on the outcome.

The winter he named

The same habit went somewhere much less congenial. In 1983 he cowrote the paper that introduced the concept of nuclear winter, the global cooling that would follow a nuclear war, and he coauthored The Cold and the Dark: The World After Nuclear War the following year (Britannica).

Read that next to the Venus work and the method is identical: an atmosphere, a model of how light moves through it, and an arithmetic answer whose political consequences nobody had requested. A greenhouse traps heat. Soot and dust thrown into the upper air keep sunlight out. The physics is not exotic; the willingness to publish it was.

This is where the comfortable story gets him wrong. The popularizer did not take a holiday from the science to talk to the public. The public argument and the journal paper were the same act, because a finding about the whole species has no meaning if only the species’ specialists know it. Sagan’s flair was not decoration on top of the work. It was how the work reached the people whose lives it described.

He was also, by his own lights, an advocate: he campaigned for nuclear disarmament and against the Strategic Defense Initiative, and he spent much of his career pressing the case for SETI when the case was unfashionable (Britannica). A reader who dislikes advocacy in science should sit with that. A reader who admires it should notice that the book he published in his last year, The Demon-Haunted World, is a manual for catching precisely the errors that advocacy invites, and that he wrote it while making the case.

The plaque, the record, the speck

He helped choose the Viking landing sites on Mars, and he codesigned the messages from Earth that rode out of the solar system on Pioneer and Voyager (Britannica). Those messages are a strange artifact: a greeting addressed to nobody in particular, written on the assumption that the reader will not share our senses, our mathematics, or our idea of a joke. To design one you have to ask what is true about us that could survive translation.

Then, in 1990, at his suggestion, Voyager 1 turned around and took a picture of home from about 6.4 billion kilometres away. Earth occupies a crescent 0.12 of a pixel (The Planetary Society). The image is not beautiful in the way a photograph is usually beautiful. It is a bright grain in a band of scattered light, and everything anyone has ever done is inside it.

What he wrote about that grain is the passage he is now mostly remembered for:

Look again at that dot. That’s here. That’s home. That’s us.

That is the opening of the passage, from Pale Blue Dot, 1994. The lines that follow are a list of what happens on the dot: every confident religion, every hunter and forager, every hero and coward, every king and peasant, every saint and sinner. It is a memento mori at planetary scale, and it works on the reader for the same reason the Stoic version works: not because it makes human beings small, but because it makes their quarrels proportionate.

The medal and the chair

Here is a fact I did not expect, sitting in the same reference book: in 1994 the National Academy of Sciences gave Sagan its Public Welfare Medal, its highest award for the application of science to the public welfare (The Planetary Society), and he was never elected a member of the National Academy of Sciences itself (Britannica).

Both halves are on the record, side by side, and neither explains the other. The record does not say why the vote went the way it did, so I will not invent a reason. What the pair does show is that the profession had two ledgers, one for the work and one for the explaining, and the entries did not have to agree. Sagan got the medal and not the chair, and he went on working in the same direction until the last year of his life, when The Demon-Haunted World came out and argued for a candle in a different kind of dark (The Planetary Society).

What he actually asked of a reader

Cosmos was seen by more than 500 million people in 60 countries (NASA), and the book of the same name became a best seller (Britannica). It is easy to read that as an audience figure and stop. But the series is not a spectacle. It is a tutorial in a specific posture, and Sagan named the two halves of it plainly:

Imagination will often carry us to worlds that never were. But without it we go nowhere.

The nature of life on Earth and the search for life elsewhere are two sides of the same question – the search for who we are.

Both passages are quoted in NASA’s “Carl Sagan (1934-1996)” profile.

The two quotations are a matched pair, and they are the whole argument. Imagination without evidence drifts into fantasy; evidence without imagination never asks a question worth the instrument. He wanted both, held together, in the same mind, on the same afternoon. In an era that rewards the first without the second and the second without the first, and calls each of them a position, that is not a soft request.

He said the conclusion himself, in what may be his most quoted sentence: we are a way for the cosmos to know itself (The Planetary Society). It is a beautiful sentence and it is also a job description. Something in the universe has become able to check its own answers. That capacity is not distributed evenly, is not permanent, and is not self-maintaining.

The practice

Sagan’s birthday is a poor occasion for nostalgia, because his argument was never that the universe is wonderful. It was that wonder is the beginning of a procedure, and that the procedure can be taught, and that a civilization which stops teaching it will not notice what it has lost until the loss is expensive.

The practice, then, is small and repeatable. Take one thing you are sure about in the physical world, the kind of thing you would repeat at dinner, and ask what would have to be true for it to be false. Then look. Most of us will find that we hold a handful of beliefs about this universe in exactly the state Sagan spent his life correcting: inherited, comfortable, and never once tested against an instrument.

He would have been ninety-two today. The candle in the picture is the wrong metaphor if it means comfort. It is the right one if it means proportion: enough light to see the next step, and no honest claim to more.

Sources

PRH | huffmanwrites.org | © Philip Huffman