Carl Edward Sagan (; SAY-gən; November 9, 1934 – December 20, 1996) was an American astronomer, planetary scientist and science communicator. Initially an assistant professor at Harvard, Sagan later moved to Cornell, where he was the David Duncan Professor of Astronomy and Space Sciences and directed the Laboratory for Planetary Studies. He played an active role in the Mariner, Viking and Voyager programs. He published more than 600 scientific papers and articles and several popular science books, starting with The Cosmic Connection. He won the Pulitzer Prize for General Nonfiction for The Dragons of Eden. He is widely regarded as one of the most influential science communicators of his generation. He co-wrote and narrated the 1980 documentary series Cosmos: A Personal Voyage, which has been seen by at least 500 million people in 60 countries and won two Emmy Awards and a Peabody Award. Cosmos, the companion volume, was the bestselling science book to date. A lifelong science fiction fan, Sagan entered the genre with Contact, which was adapted as the film of the same name. He was a founding member and first president of the Planetary Society. He proposed the Pale Blue Dot photograph of Earth taken by Voyager 1. He had a lifelong interest in the possibility of extraterrestrial life and is generally credited with contributions to the Arecibo message, with a much more significant role developing the Pioneer plaques and the Voyager Golden Record, universal messages that could potentially be understood by any intelligence that might find them. He promoted skepticism and the scientific method, particularly in his penultimate book The Demon-Haunted World. He popularized a toolkit for critical thinking. He made famous the maxim "Extraordinary claims require extraordinary evidence." The phrase "Billions and billions" was attributed to him, although he never said it; he did use it as the title of his last book. Sagan received numerous awards and honors, including the NASA Distinguished Public Service Medal and the National Academy of Sciences Public Welfare Medal. He married three times and had five children. After developing myelodysplasia, Sagan died of pneumonia at the age of 62 on December 20, 1996.
Early life
Childhood
Carl Edward Sagan was born on November 9, 1934, in the Bensonhurst neighborhood of New York City's Brooklyn borough. His mother, Rachel Molly Gruber (1906–1982), was a housewife from New York City; his father, Samuel Sagan (1905–1979), was a Ukrainian-born garment worker who had emigrated from Kamianets-Podilskyi (then in the Russian Empire). Sagan was named in honor of his maternal grandmother, Chaiya Clara, who had died while giving birth to her second child; she was, in Sagan's words, "the mother she [Rachel] never knew." Sagan's family lived in a modest apartment in Bensonhurst. He later described his family as Reform Jews, one of the more liberal of Judaism's four main branches. He and his sister agreed that their father was not especially religious, but that their mother "definitely believed in God, and was active in the temple [...] and served only kosher meat." During the worst years of the Great Depression, his father worked as a movie theater usher. According to biographer Keay Davidson, Sagan traced his analytical inclinations to his mother, who had been extremely poor as a child in New York City during World War I and the 1920s, and whose later intellectual ambitions were sabotaged by her poverty, status as a woman and wife, and Jewish ethnicity. Davidson suggested she "worshipped her only son, Carl" because "he would fulfill her unfulfilled dreams." Sagan traced his sense of wonder to his father, who spent his free time giving apples to the poor or helping soothe tensions between workers and management within New York City's garment industry. Sagan said: "My parents were not scientists. They knew almost nothing about science. But in introducing me simultaneously to skepticism and to wonder, they taught me the two uneasily cohabiting modes of thought that are central to the scientific method." He described a defining moment in his development, when his parents took him to the 1939 New York World's Fair. He recalled his vivid memories of several exhibits there. One, Futurama, included a moving map, which, as he recalled, "showed beautiful highways and cloverleaves and little General Motors cars all carrying people to skyscrapers, buildings with lovely spires, flying buttresses—and it looked great!" Another involved a flashlight shining on a photoelectric cell, which created a crackling sound, and another showed how the sound from a tuning fork became a wave on an oscilloscope. He saw an exhibit of the nascent medium of television. He later wrote: "Plainly, the world held wonders of a kind I had never guessed. How could a tone become a picture and light become a noise?" Sagan saw one of the fair's most publicized events: the burial at Flushing Meadows of a time capsule, which contained mementos from the 1930s to be recovered in the far future. Davidson wrote that this "thrilled Carl." As an adult, Sagan and his colleagues would create similar time capsules to be sent out into space. During World War II, Sagan's parents worried about the fate of their European relatives, but he was generally unaware of the details of the ongoing war. He wrote, "Sure, we had relatives who were caught up in the Holocaust. Hitler was not a popular fellow in our household... but on the other hand, I was fairly insulated from the horrors of the war." His sister, Carol, said that their mother "above all wanted to protect Carl... she had an extraordinarily difficult time dealing with World War II and the Holocaust."
He wondered what the stars were, but no one could give him a clear answer. He recalled: As soon as I was old enough, my parents gave me my very first library card. I think the library was on 85th Street, an alien land. Immediately, I asked the librarian for something on stars. She returned with a picture book displaying portraits of men and women with names like Clark Gable and Jean Harlow. I complained, and for some reason then obscure to me, she smiled and found another book—the right kind of book. I opened it breathlessly and read until I found it. The book said something astonishing, a very big thought. It said that the stars were suns, but very far away. The Sun was a star, but close up... I was innocent of the notion of the inverse square law for light propagation. I had not the ghost of a chance of calculating the distance to the stars. But I could tell that if the stars were suns, they had to be very far away—farther away than 85th Street, farther away than Manhattan, farther away, probably, than New Jersey. The Cosmos was much bigger than I had guessed. He said: "The scale of the universe suddenly opened up to me. It was a kind of religious experience. There was a magnificence to it, a grandeur, a scale which has never left me. Never ever left me." At age six or seven, he and a close friend took trips to the American Museum of Natural History. They were impressed by the displays of dinosaur fossils and nature dioramas and by the Hayden Planetarium. Sagan's parents nurtured his growing interest in science, buying him chemistry sets and reading matter. Per biographer Ray Spangenburg, Sagan's desire to understand the Cosmos became a "driving force in his life, a continual spark to his intellect, and a quest that would never be forgotten." His fascination with outer space deepened after reading Edgar Rice Burroughs's John Carter of Mars books. In 1947, he discovered Astounding Science Fiction, which introduced him to more hard science fiction speculations: "Each month I eagerly awaited the arrival of Astounding. I read Verne and Wells, read, cover‐to‐cover, the first two science‐fiction anthologies that I was able to find, devised scorecards, similar to those I was fond of making for baseball, on the quality of the stories I read. Many ranked high in asking interesting questions but low in answering them."
Education
Sagan attended David A. Boody Junior High School, in his native Bensonhurst and had his bar mitzvah when he turned 13. In 1948, when he was 14, his father's work took the family to Rahway, New Jersey, where he attended Rahway High School. He was a straight-A student but was bored because his classes did not challenge him and his teachers did not inspire him. His teachers realized this and tried to convince his parents to send him to a private school, with an administrator telling them, "This kid ought to go to a school for gifted children, he has something really remarkable." However, his parents could not afford to do so. Sagan became president of the school's chemistry club, and set up his own laboratory at home. He taught himself about molecules by making cardboard cutouts to help him visualize how they were formed: "I found that about as interesting as doing [chemical] experiments." He was mostly interested in astronomy, studying it in his spare time. In his junior year of high school, he discovered that professional astronomers were paid for doing something he always enjoyed. "That was a splendid day—when I began to suspect that if I tried hard I could do astronomy full-time." Before finishing high school, Sagan entered an essay writing contest in which he explored the idea that human contact with advanced extraterrestrials might be as disastrous for people on Earth as Native Americans' first contact with Europeans had been for Native Americans. The subject was considered controversial, but his rhetorical skill won over the judges and they awarded him first prize. When he was about to graduate from high school, his classmates voted him "most likely to succeed" and put him in line to be valedictorian. In 1950, Sagan wrote the essay "Space, Time, and the Poet" for his high school newspaper. In it, he mused on man's place in the universe as expressed by poets like T. S. Eliot and Alfred, Lord Tennyson and "the work containing perhaps the greatest poetry—the Bible." He graduated from Rahway High School in 1951. He attended the University of Chicago as it was one of the few colleges he had applied to that would accept a 16 year old. Robert M. Hutchins, its chancellor, had recently retooled the College of the University of Chicago into an "ideal meritocracy" built on Great Books, Socratic dialogue, comprehensive examinations, and early entrance to college. He joined the Ryerson Astronomical Society. He wrote "College was the fulfillment of my dreams. I found teachers who not only understood science, but were actually able to explain it. … I was a physics student in a department orbiting around Enrico Fermi; I discovered what true mathematical elegance is from Subrahmanyan Chandrasekhar; I was given the chance to talk chemistry with Harold Urey; over summers I was apprenticed in biology to H. J. Muller at Indiana University; and I learned planetary astronomy from its only full-time practitioner at the time, G. P. Kuiper." The Miller-Urey experiment, conducted in 1952, sparked his interest in the origin of life. Under Urey, he wrote "Radiation and the Origin of the Gene." He recalled that "science was presented as an integral part of the gorgeous tapestry of human knowledge. It was considered unthinkable for an aspiring physicist not to know Plato, Aristotle, Bach, Shakespeare, Gibbon, Malinowski and Freud—among many others." He was awarded a Bachelor of Liberal Arts with general and special honors in what he quipped was "nothing." In 1955, he earned a Bachelor of Science in physics. He went on to do graduate work at the University of Chicago, earning a Master of Science in physics in 1956 and a Doctor of Philosophy in astronomy and astrophysics in 1960. His doctoral thesis, under the direction of Kuiper, was "Physical Studies of the Planets". During his graduate studies, he spent summers working with Kuiper, as well as chemist Melvin Calvin and physicist George Gamow. He credited Kuiper with teaching him back-of-the-envelope calculations: "A possible explanation to a problem occurs to you, you pull out an old envelope, appeal to your knowledge of fundamental physics, scribble a few approximate equations on the envelope, and see if your answer comes anywhere near explaining your problem. If not, you look for a different explanation. It cut through nonsense like a knife through butter." In 1958, Sagan and Kuiper worked on the classified military Project A119, a secret United States Air Force plan to detonate a nuclear warhead on the Moon and document its effects. Sagan had a Top Secret clearance at the Air Force and a Secret clearance with NASA. In 1999, an article published in the journal Nature revealed that Sagan had included the classified titles of two Project A119 papers in his 1959 application for a scholarship to University of California, Berkeley. A follow-up letter to the journal by project leader Leonard Reiffel confirmed Sagan's security leak.
Career and research
From 1960 to 1962, Sagan was a Miller Fellow at the University of California, Berkeley. Meanwhile, he published an article in 1961 in the journal Science on the atmosphere of Venus, while also working with NASA's Mariner 2 team, and served as a "Planetary Sciences Consultant" to the RAND Corporation. After the publication of Sagan's Science article, in 1961, Harvard University astronomers Fred Whipple and Donald Menzel offered Sagan the opportunity to give a colloquium at Harvard and subsequently offered him a lecturer position at the institution. Sagan instead asked to be made an assistant professor, and eventually Whipple and Menzel were able to convince Harvard to offer Sagan the assistant professor position he requested. Sagan lectured, performed research, and advised graduate students at the institution from 1963 until 1968, as well as working at the Smithsonian Astrophysical Observatory, also located in Cambridge, Massachusetts. In 1968, Sagan was denied academic tenure at Harvard. He later indicated that the decision was very unexpected. The denial has been blamed on several factors, including that he focused his interests too broadly across a number of areas (while the norm in academia is to become a renowned expert in a narrow specialty), and perhaps because of his well-publicized scientific advocacy, which some scientists perceived as borrowing the ideas of others for little more than self-promotion. An advisor from his years as an undergraduate student, Harold Urey, wrote a letter to the tenure committee recommending strongly against tenure for Sagan. Long before the ill-fated tenure process, Cornell University astronomer Thomas Gold had courted Sagan to move to Ithaca, New York, and join the recently hired astronomer Frank Drake among the faculty at Cornell. Following the denial of tenure from Harvard, Sagan accepted Gold's offer and remained a faculty member at Cornell for nearly 30 years until his death in 1996. Unlike Harvard, the smaller and more laid-back astronomy department at Cornell welcomed Sagan's growing celebrity status. Following two years as an associate professor, Sagan became a full professor at Cornell in 1970 and directed the Laboratory for Planetary Studies there. From 1972 to 1981, he was associate director of the Center for Radiophysics and Space Research (CRSR) at Cornell. In 1976, he became the David Duncan Professor of Astronomy and Space Sciences, a position he held for the remainder of his life. Sagan was associated with the U.S. space program from its inception. From the 1950s onward, he worked as an advisor to NASA, where one of his duties included briefing the Apollo astronauts before their flights to the Moon. Sagan contributed to many of the robotic spacecraft missions that explored the Solar System, arranging experiments on many of the expeditions. He often challenged the decisions to fund the Space Shuttle and the International Space Station at the expense of further robotic missions. Sagan assembled the first physical message that was sent into space: a gold-plated plaque, attached to the space probe Pioneer 10, launched in 1972. Pioneer 11, also carrying another copy of the plaque, was launched the following year. He continued to refine his designs. He contributed to the Voyager Golden Record, a sample of the sights and sounds of Earth sent with the Voyager space probes in 1977. Among much else, it features music by Bach, Beethoven and Chuck Berry.
Scientific achievements
Former student David Morrison described Sagan as "an 'idea person' and a master of intuitive physical arguments and 'back of the envelope' calculations", and Gerard Kuiper said that "Some persons work best in specializing on a major program in the laboratory; others are best in liaison between sciences. Dr. Sagan belongs in the latter group." Sagan's contributions were central to the discovery of the high surface temperatures of the planet Venus. In the early 1960s no one knew for certain the basic conditions of Venus' surface, and Sagan listed the possibilities in a report later depicted for popularization in a Time Life book Planets. His own view was that Venus was dry and very hot as opposed to the balmy paradise others had imagined. He had investigated radio waves from Venus and concluded that there was a surface temperature of 500 °C (900 °F). As a visiting scientist to NASA's Jet Propulsion Laboratory, he contributed to the first Mariner missions to Venus, working on the design and management of the project. Mariner 2 confirmed his conclusions on the surface conditions of Venus in 1962. Sagan was among the first to hypothesize that Saturn's moon Titan might possess oceans of liquid compounds on its surface and that Jupiter's moon Europa might possess subsurface oceans of water. This would make Europa potentially habitable. Europa's subsurface ocean of water was later indirectly confirmed by the spacecraft Galileo. The mystery of Titan's reddish haze was also solved with Sagan's help. The reddish haze was revealed to be due to complex organic molecules constantly raining down onto Titan's surface. Sagan further contributed insights regarding the atmospheres of Venus and Jupiter, as well as seasonal changes on Mars. He also perceived climate change as a growing, man-made danger and likened it to the natural development of Venus into a hot, life-hostile planet through a kind of runaway greenhouse effect. He testified to the US Congress in 1985 that the greenhouse effect would change the Earth's climate system. Sagan and his Cornell colleague Edwin Ernest Salpeter speculated about life in Jupiter's clouds, given the planet's dense atmospheric composition rich in organic molecules. He studied the observed color variations on Mars' surface and concluded that they were not seasonal or vegetational changes as most believed, but shifts in surface dust caused by windstorms. He argued in favor of the hypothesis, which has since been accepted, that the high surface temperatures of Venus are the result of the greenhouse effect. Sagan is also known for his research on the possibilities of extraterrestrial life, including experimental demonstration of the production of amino acids from basic chemicals by radiation. As of 2017, Sagan is the most cited SETI scientist and one of the most cited planetary scientists. He edited Icarus from 1975 to 1979. In 1980, he cofounded the Planetary Society.
Science popularization
Sagan wrote the Encyclopædia Britannica entry on Life, later updated by his first wife, the biologist Lynn Margulis. Sagan's first popular science book was The Cosmic Connection. He introduced the Cosmic Calendar in The Dragons of Eden, which won the 1978 Pulitzer Prize for General Nonfiction. He delivered the 1977 Royal Institution Christmas Lectures on The Planets.
Sagan and Ann Druyan co-wrote the 13-part PBS documentary Cosmos: A Personal Voyage. It drew on earlier documentaries, notably Jacob Bronowski's The Ascent of Man. The production involved the recreation of the Library of Alexandria. It covered an array of scientific subjects, including the evolution of stars and how it is linked to the evolution of life. Frederic Golden wrote "The series' name comes from the Greek word for the ordered universe, the antithesis of chaos. It is an apt choice. Cosmos is nothing less than Sagan's attempt to make sense out of what is for many people the hopelessly baffling world of 20th century science. To unfold his story he roves through two millennia of scientific progress, often shuttling back and forth over the centuries like some Wellsian time traveler. One moment he is seated in a cafe on the Aegean island of Samos, home of Pythagoras and Aristarchus, explaining the first stirrings of Greek scientific prowess. At another moment, he is strolling through the venerable Cavendish Laboratories of England's Cambridge University, recounting the birth of modern atomic physics. Sagan makes science as palatable as the apple pie he lovingly cuts up in a Cambridge University dining room in order to make a point about matter." He offers an optimistic and a perspective of humans' place on Earth, arguing that "We are a way for the cosmos to know itself." Cosmos has been seen by at least 500 million people across 60 countries, making it the most widely watched series in the history of American public television until Ken Burns's The Civil War in 1990. Cosmos won an Emmy and a Peabody. It featured music by Bach, Vivaldi, Vangelis and others. The accompanying book was well received. James Michener wrote "Mr. Sagan's essay, a spin-off from his hugely successful television show, is a cleverly written, imaginatively illustrated summary of his geological, anthropological, biological, historical and astronomical ruminations about our universe. His references comprise the entire scope of human history. His treatment, necessarily abbreviated, is highly personal. He is always readable, and because his mind ranges so far and wide, he seems exactly the right man for the job." He wrote the introduction to Stephen Hawking's bestseller A Brief History of Time. In 1988, Magnus Magnusson moderated a discussion between Sagan, Hawking and Arthur C. Clarke, God, the Universe and Everything Else. He wrote a sequel to Cosmos, Pale Blue Dot. The title refers to the view of Earth from the Voyager spacecraft. Sagan said that there were at least two reasons for scientists to share the purposes and findings of science. Simple self-interest was one: much of the funding for science came from the public, and the public therefore had the right to know how the money was being spent. If scientists increased public admiration for science, there was a good chance of having more public supporters. The other reason was the joy of communicating one's own excitement about science to others. He wrote: "Among the best contemporary scientist-popularizers, I think of Stephen Jay Gould, E. O. Wilson, Lewis Thomas and Richard Dawkins in biology; Steven Weinberg, Alan Lightman and Kip Thorne in physics; Roald Hoffmann in chemistry; and the early works of Fred Hoyle in astronomy. (And while requiring calculus, the most consistently exciting, provocative, and inspiring science popularization of the last few decades seems to me to be Volume 1 of Richard Feynman's Introductory Lectures on Physics.)"
Science fiction Sagan wrote that science fiction led him to science. He added that, while most of the science fiction he read in his youth didn't hold up, "the best of science fiction remains very good indeed. There are stories that are so tautly constructed, so rich in the accommodating details of an unfamiliar society that they sweep me along before I have even a chance to be critical. Such works include Robert Heinlein's The Door into Summer; Alfred Bester's The Stars My Destination and his The Demolished Man; Jack Finney's Time and Again; Frank Herbert's Dune, and Walter M. Miller's A Canticle for Leibowitz." Sagan was acquainted with science fiction fandom through his friendship with Isaac Asimov, and he spoke at the Nebula Awards ceremony in 1969. Asimov described Sagan as one of only two people he ever met whose intellect surpassed his own, the other being computer scientist and artificial intelligence expert Marvin Minsky. Sagan briefly served as an adviser on Stanley Kubrick's 2001: A Space Odyssey. He proposed that the film suggest, rather than depict, extraterrestrial superintelligence. In 1971, he participated in a panel on Mars with Ray Bradbury, Arthur C. Clarke, Bruce C. Murray and Walter Sullivan, published as Mars and the Mind of Man. Sagan turned his pen to science fiction with Contact. He needed a way for his heroine, Ellie Arroway, to get from Earth to Vega, so he asked his friend Kip Thorne for advice on the physics of wormholes. This led to original research by Thorne on closed timelike curves.
Skepticism Sagan promoted scientific skepticism against pseudoscience. He credited Martin Gardner's Fads and Fallacies in the Name of Science and Charles Mackay's Extraordinary Popular Delusions and the Madness of Crowds with teaching him critical thinking. In 1974, he challenged Immanuel Velikovsky to a debate. He was a critic of practices like crystal healing and astrology. In a column for Parade, he proposed a "Baloney Detection Kit", a phrase coined by Arthur Felberbaum, a friend of his wife Ann Druyan. He expanded on it in his penultimate book, The Demon-Haunted World. He lamented the fact that most newspapers had a daily column on astrology and very few had even a weekly column on astronomy. To mark the tenth anniversary of Sagan's death, David Morrison, a former student of Sagan, recalled "Sagan's immense contributions to planetary research, the public understanding of science, and the skeptical movement" in Skeptical Inquirer. He taught a Senior Seminar on "Critical Thinking".
One of his most famous quotations, "extraordinary claims require extraordinary evidence", is called the "Sagan standard" by some. It was based on a nearly identical statement by fellow founder of the Committee for the Scientific Investigation of Claims of the Paranormal, Marcello Truzzi, "An extraordinary claim requires extraordinary proof." This idea had been aphorized in Théodore Flournoy's From India to the Planet Mars (1899) from a longer quote by the French mathematician and astronomer Pierre-Simon Laplace as the Principle of Laplace: "The weight of the evidence should be proportioned to the strangeness of the facts." He noted that science's predictive power distinguished it from pseudoscience: "If you want to know when the next eclipse of the Sun will be, you might try magicians and mystics, but you'll do much better with scientists. They will tell you where on Earth to stand, when you have to be there, and whether it will be a partial eclipse, a total eclipse, or an annular eclipse. They can routinely predict a solar eclipse, up to the minute, a century in advance. You can go to the witch doctor to lift the spell that causes your pernicious anemia, or you can take Vitamin B12. If you want to save your child from polio, you can pray or you can inoculate."
Other interests In his later years, Sagan proposed organizing a search for near-Earth objects (NEOs) that might impact the Earth but postponing the development of the technological methods needed to defend against them. He argued that all of the numerous methods proposed to alter the orbit of an asteroid, including the employment of nuclear detonations, created a deflection dilemma: if the ability to deflect an asteroid away from the Earth exists, then one would also have the ability to divert a non-threatening object towards Earth, creating an immensely destructive weapon. In a 1994 paper he co-authored, he ridiculed a three-day-long "Near-Earth Object Interception Workshop" held by Los Alamos National Laboratory (LANL) in 1993 that did not, "even in passing" state that such interception and deflection technologies could have these "ancillary dangers." Sagan remained hopeful that the natural NEO impact threat and the intrinsically double-edged essence of the methods to prevent these threats would serve as a "new and potent motivation to maturing international relations." Later acknowledging that, with sufficient international oversight, in the future a "work our way up" approach to implementing nuclear explosive deflection methods could be fielded, and when sufficient knowledge was gained, to use them to aid in mining asteroids. His interest in the use of nuclear detonations in space grew out of his work in 1958 for the Armour Research Foundation's Project A119, concerning the possibility of detonating a nuclear device on the lunar surface. He was an advocate for basic research, pointing out that it might prove to have practical applications in the future: "Maxwell wasn't thinking of radio, radar, and television when he first scratched out the fundamental equations of electromagnetism; Newton wasn't dreaming of space flight or communications satellites when he first understood the motion of the Moon; Roentgen wasn't contemplating medical diagnosis when he investigated a penetrating radiation so mysterious he called it 'X-rays'; Curie wasn't thinking of cancer therapy when she painstakingly extracted minute amounts of radium from tons of pitchblende; Fleming wasn't planning on saving the lives of millions with antibiotics when he noticed a circle free of bacteria around a growth of mold; Watson and Crick weren't imagining the cure of genetic diseases when they puzzled over the X-ray diffractometry of DNA…"
Sagan's number Sagan's number is the number of stars in the observable universe. This number is reasonably well defined, because it is known what stars are and what the observable universe is, but its value is highly uncertain.
In 1980, Sagan estimated it to be 10 sextillion in short scale (1022). In 2003, it was estimated to be 70 sextillion (7 × 1022). In 2010, it was estimated to be 300 sextillion (3 × 1023).
"Billions and billions" After Cosmos aired, Sagan became associated with the catchphrase "billions and billions", although he never actually said it. He rather used the term "billions upon billions." Richard Feynman used the phrase "billions and billions" many times in his Lectures on Physics. However, Sagan's frequent use of the word billions and distinctive delivery emphasizing the "b" (which he did intentionally, in place of more cumbersome alternatives such as "billions with a 'b'", in order to distinguish the word from "millions") was spoofed by Johnny Carson. Sagan was a friend of Carson's and a frequent guest on the Tonight Show. Other comedians followed Carson's lead, including Gary Kroeger, Mike Myers, Bronson Pinchot, Penn Jillette, Harry Shearer, and others. Frank Zappa satirized the line in the song "Be in My Video". Sagan took this all in good humor, and his final book was titled Billions and Billions, which opened with a tongue-in-cheek discussion of this catchphrase, observing that Carson was an amateur astronomer and that Carson's comic caricature often included real science. In 1993, engineers at Apple Computer code-named the Power Macintosh 7100 "Carl Sagan" in the hope that Apple would make "billions and billions". The name was only used internally, but Sagan was concerned that it would become a product endorsement and sent Apple a cease-and-desist letter. Apple complied, but engineers retaliated by changing the internal codename to "BHA" for "Butt-Head Astronomer." In November 1995, after further legal battle, an out-of-court settlement was reached and Apple's office of trademarks and patents released a conciliatory statement that "Apple has always had great respect for Dr. Sagan. It was never Apple's intention to cause Dr. Sagan or his family any embarrassment or concern." As a humorous tribute to Sagan and his association with the catchphrase "billions and billions", a sagan has been defined as a unit of measurement equivalent to a very large number of anything.
Criticisms While Sagan was widely adored by the general public, his reputation in the scientific community was more polarized. Critics sometimes characterized his work as fanciful, non-rigorous, and self-aggrandizing, and others complained in his later years that he neglected his role as a faculty member to foster his celebrity status. One of Sagan's harshest critics, Harold Urey, felt that Sagan was getting too much publicity for a scientist and was treating some scientific theories too casually. Urey and Sagan were said to have different philosophies of science, according to Davidson. While Urey was an "old-time empiricist" who avoided theorizing about the unknown, Sagan was by contrast willing to speculate openly about such matters. Fred Whipple wanted Harvard to keep Sagan there, but learned that because Urey was a Nobel laureate, his opinion was an important factor in Harvard denying Sagan tenure. Sagan's Harvard friend Lester Grinspoon also stated: "I know Harvard well enough to know there are people there who certainly do not like people who are outspoken." Grinspoon added:
Wherever you turned, there was one astronomer being quoted on everything, one astronomer whose face you were seeing on TV, and one astronomer whose books had the preferred display slot at the local bookstore. Some, like Urey, later believed that Sagan's popular brand of scientific advocacy was beneficial to the science as a whole. Urey especially liked Sagan's 1977 book The Dragons of Eden and wrote Sagan with his opinion: "I like it very much and am amazed that someone like you has such an intimate knowledge of the various features of the problem... I congratulate you... You are a man of many talents." Sagan was accused of borrowing some ideas of others for his own benefit and countered these claims by explaining that the misappropriation was an unfortunate side effect of his role as a science communicator and explainer, and that he attempted to give proper credit whenever possible.
Social concerns
At the height of the Cold War, Sagan became involved in nuclear disarmament efforts by promoting hypotheses on the effects of nuclear war, when Paul Crutzen's "Twilight at Noon" concept suggested that a substantial nuclear exchange could trigger a nuclear twilight and upset the delicate balance of life on Earth by cooling the surface. In 1983, he was one of five authors—the "S"—in the follow-up "TTAPS" model (as the research article came to be known), which contained the first use of the term "nuclear winter", which his colleague Richard P. Turco had coined. In 1984, he co-authored the book The Cold and the Dark: The World after Nuclear War and in 1990, the book A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race, which explains the nuclear-winter hypothesis and advocates nuclear disarmament. Sagan received a great deal of skepticism and disdain for the use of media to disseminate a very uncertain hypothesis. A personal correspondence with nuclear physicist Edward Teller around 1983 began amicably, with Teller expressing support for continued research to ascertain the credibility of the winter hypothesis. However, Sagan and Teller's correspondence would ultimately result in Teller writing: "A propagandist is one who uses incomplete information to produce maximum persuasion. I can compliment you on being, indeed, an excellent propagandist, remembering that a propagandist is the better the less he appears to be one." Biographers of Sagan would also comment that from a scientific viewpoint, nuclear winter was a low point for Sagan, although, politically speaking, it popularized his image among the public. Sagan believed that the Drake equation suggested that a large number of extraterrestrial civilizations would form, but that the lack of evidence of such civilizations highlighted by the Fermi paradox suggests technological civilizations tend to self-destruct. This stimulated his interest in identifying and publicizing ways that humanity could destroy itself, with the hope of avoiding such a cataclysm and eventually becoming a spacefaring species. Sagan's deep concern regarding the potential destruction of human civilization in a nuclear holocaust was conveyed in a memorable cinematic sequence in the final episode of Cosmos, "Who Speaks for Earth?" Sagan had already resigned from the Air Force Scientific Advisory Board's UFO-investigating Condon Committee and voluntarily surrendered his top-secret clearance in protest over the Vietnam War. Following his marriage to his third wife (novelist Ann Druyan) in June 1981, Sagan became more politically active—particularly in opposing escalation of the nuclear arms race under President Ronald Reagan.
In March 1983, Reagan announced the Strategic Defense Initiative—a multibillion-dollar project to develop a comprehensive defense against attack by nuclear missiles, which was quickly dubbed "Star Wars". Sagan, along with other scientists, spoke out against the project, arguing that it was technically impossible to develop a system with the level of perfection required, and far more expensive to build such a system than it would be for an enemy to defeat it through decoys and other means—and that its construction would seriously destabilize the "nuclear balance" between the United States and the Soviet Union, making further progress toward nuclear disarmament impossible. When Soviet leader Mikhail Gorbachev declared a unilateral moratorium on the testing of nuclear weapons, which would begin on August 6, 1985—the 40th anniversary of the atomic bombing of Hiroshima—the Reagan administration dismissed the dramatic move as nothing more than propaganda and refused to follow suit. In response, US anti-nuclear and peace activists staged a series of protest actions at the Nevada Test Site, beginning on Easter Sunday in 1986 and continuing through 1987. Hundreds of people in the "Nevada Desert Experience" group were arrested, including Sagan, who was arrested on two separate occasions as he climbed over a chain-link fence at the test site during the underground Operation Charioteer and United States's Musketeer nuclear test series of detonations. He was an advocate for free speech and civil liberties. His professor Edward Condon, during the McCarthy era, was accused by HUAC of being a "revolutionary in physics". Sagan quotes Condon as replying: "I believe in Archimedes' Principle, formulated in the third century B. C. I believe in Kepler's laws of planetary motion, discovered in the seventeenth century. I believe in Newton's laws…", going on to invoke Bernoulli, Fourier, Ampère, Boltzmann, and Maxwell. The committee was not amused. "But the most they were able to pin on Condon, as I recall, was that in high school he had a job delivering a socialist newspaper door-to-door on his bicycle." When visiting the Soviet Union, he and Druyan would smuggle in banned books.
In a speech given at Monticello on July 4, 1992, Sagan emphasized the importance of science to Thomas Jefferson, John Adams, Benjamin Franklin and democracy in America: It is a fact of life on our beleaguered little planet that widespread torture, famine and governmental criminal corruption are more likely to be found in tyrannical than in democratic governments. Why? Because the rulers of the former are much less likely to be thrown out of office for their misdeeds than the rulers of the latter. This is the error correction machinery in politics. He notes that "New ideas, invention, and creativity in general, always spearhead a new kind of freedom—a breaking out from hobbling constraints. Freedom is a prerequisite for continuing the delicate experiment of science—which is one reason the Soviet Union could not remain a totalitarian state and remain technically competitive. At the same time, science—or rather its delicate mix of o