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Carl Sagan's criticisms of Worlds in Collision

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Worlds in Collision, the subject of Carl Sagan's criticisms
Scientists Confront Velikovsky, in which Carl Sagan's AAAS paper first appeared.
Broca's Brain also includes a version of Carl Sagan's AAAS paper.

Astronomer Carl Sagan criticized the ideas in Immanuel Velikovsky's book, Worlds in Collision, presenting them as "Ten Problems" in:


Sagan's "Ten Problems" summarized

  • Introduction
  • The Uniformitarians and the Catastrophists
  • The Method of Concordances in Myth and Legend
  • Velikovsky's Principal Hypothesis
  • Problem I. The Ejection of Venus by Jupiter
  • Problem II. Repeated Collisions among the Earth, Venus, and Mars
  • Problem III. The Earth's Rotation
  • Problem IV. Terrestrial Geology and Lunar Craters
  • Problem V. Chemistry and Biology of the Terrestrial Planets
  • Problem VI. Manna
  • Problem VII. The Clouds of Venus
  • Problem VIII. The Temperature of Venus
  • Problem IX. The Craters of Venus
  • Problem X. The Circularization of the Orbit of Venus and Nongravitational Forces in the Solar System
  • Some Other Problems Conclusions
  • Acknowledgments
  • Appendixes:
  1. Simple Collision Physics Discussion of the Probability of a Recent Collision with the Earth by a Massive Member of the Solar System;
  2. Consequences of a Sudden Deceleration of the Earth's Rotation;
  3. Present Temperature of Venus If Heated by a Close Passage to the Sun;
  4. Magnetic Field Strengths Necessary to Circularize an Eccentric Cometary Orbit

Problem I. The Ejection of Venus by Jupiter

Sagan writes:

"Velikovsky's hypothesis begins with an event which has never been observed by astronomers and which is inconsistent with much that we know about planetary and cometary physics, namely, the ejection of an object of planetary dimensions from Jupiter. [..] any event which would have ejected a comet or a planet from Jupiter would have brought it to a temperature of at least several thousands of degrees [..] The likelihood of a planet, much less an icy comet, surviving ejection seems small. [..] Another problem is that the escape velocity [..] There is only a narrow and therefore unlikely range of velocities which is consistent with Velikovsky's hypothesis [..] A further problem is that the mass of Venus is very large [..] The total kinetic energy required to propel Venus to Jovian escape velocity [..] is equivalent to all the energy radiated by the sun to space in an entire year [..] an ejection event vastly more powerful than anything on the sun"[1]

Problem II. Repeated Collisions among the Earth, Venus, and Mars

Sagan writes:

"That a comet may strike our planet is not very probable, but the idea is not absurd" (page 40). This is precisely correct: it only remains to calculate the probabilities, which Velikovsky has unfortunately left undone.
"Fortunately, the relevant physics is extremely simple and can be performed to order of magnitude even without any consideration of gravitation. [..] the odds against a collision with the Earth in any given year is 3 x 107 to one; the odds against it in any given millennium are thirty thousand to one. But Velikovsky has (see, e.g., page 388) not one but five or six near collisions [..] If the probabilities are independent, then the joint probability of five such encounters in the same millennium is on the short side of (3 x 107/103)-5 = (3 x 104)-5= 4.1 x 1023, or odds of almost a hundred billion trillion to one [..] Hypotheses with such small odds in their favor are usually said to be untenable"[1]

Problem III. The Earth's Rotation

Sagan writes:

"Much of the indignation directed toward Worlds in Collision seems to have arisen from Velikovsky's interpretation of the story of Joshua and related legends as implying that the Earth's rotation was once braked to a halt. [..] we see in Appendix 2 that the energy required to brake the Earth is not enough to melt it, although it would result in a noticeable increase in temperature: the oceans would have been raised to the boiling point of water, an event which seems to have been overlooked by Velikovsky's ancient sources.
"Perhaps the most serious objection is rather at the other end. How does the Earth get started up again, rotating at approximately the same rate of spin? The Earth cannot do it by itself, because of the law of the conservation of angular momentum. [..] Velikovsky is vague about the mechanism which is supposed to have braked the Earth's rotation".[1]

Problem IV. Terrestrial Geology and Lunar Craters

Sagan writes:

"Velikovsky believes (page 115) that reversals of the geomagnetic field are produced by cometary close approaches. Yet the record from rock magnetization is clear - such reversals occur about every million years, though not in the last few thousand; and they recur more or less like clockwork. [..]"
"Velikovsky's contention that mountain-building occurred a few thousand years ago is belied by all the geological evidence which puts those times at tens of millions of years ago and more [..]"
"Velikovsky believes that the close passage of Venus or Mars to the Earth would have produced tides at least miles high (page 70 and 71); in fact, if these planets were ever tens of thousands of kilometers away, as he seems to think, the tides, both of water and of the solid body of our planet, would be hundreds of miles high. [..] To the best of my knowledge, there is no geological evidence for a global inundation of all parts of the world either in the eighth or in the fif teenth centuries B.C"[1]

Problem V. Chemistry and Biology of the Terrestrial Planets

Sagan writes:

  • "Even stranger are Velikovsky's views on extraterrestrial life. He believes that much of the "vermin," and particularly the flies referred to in Exodus, really fell from his comet - although he hedges on the extraterrestrial origin of frogs.. [..] The idea that of all the organisms on the Earth, flies alone are of extraterrestrial origin is curiously reminiscent of Martin Luther's exasperated conclusion that, while the rest of life was created by God, the fly must have been created by the Devil because there is no conceivable practical use for it. [..] Next there is the problem of fly ablation. Small flies have just the same mass and dimensions as small meteors, which are burned up at an altitude of about 100 kilometers when they enter the Earth's atmosphere on cometary trajectories."[1]

Problem VI. Manna

Sagan writes:

"How much manna is required to feed the hundreds of thousands of Children of Israel for forty years (see Exodus 16:35)? [..] during the forty years of wandering, the whole Earth must have accumulated several times 1018 gm of manna, or enough to cover the entire surface of the planet with manna to a depth of about an inch [..] we find that the mass of manna distributed to the inner solar system by this event is larger than 1028 gm [..] Therefore, the mass of the comet must be much larger than 1031 gm. This is the mass of Jupiter."[1]

Problem VII. The Clouds of Venus

Sagan writes:

"Velikovsky's prognostication that the clouds of Venus were made of carbohydrates has many times been hailed as an example of a successful scientific prediction. [..] Moreover, the question of the composition of the Venus clouds - a major enigma for centuries - has recently been solved (Young and Young, 1973; Sill, 1972; Young, 1973; Pollack et al., 1974). The clouds of Venus are composed of an approximately 75 percent solution of sulfuric acid. [..] These observed features are inconsistent with the hypothesis of hydrocarbon or carbohydrate clouds. [..] In summary, Velikovsky's idea that the clouds of Venus are composed of hydrocarbons or carbohydrates is neither original nor correct. The "crucial test" fails."[1]

Problem VIII. The Temperature of Venus

Sagan writes:

"[Velikovsky] baldly states, "Mars emits more heat than it receives from the sun," in apparent consistency with his collision hypothesis. This statement is, however, dead wrong."
"Velikovsky nowhere states the temperature he believed Venus to be at in 1950. On page 77 he says vaguely that the comet that later became Venus was in a state of "candescence.. [..] Velikovsky writes in the 1965 preface that his claim of a high surface temperature was "in total disagreement with what was known in 1946." This turns out to be not quite the case. The dominant figure of Rupert Wildt again looms over the astronomical side of Velikovsky's hypothesis. Wildt, who, unlike Velikovsky, understood the nature of the problem, predicted correctly that Venus and not Mars would be "hot." In a 1940 paper in the Astrophysical Journal, Wildt argued that the surface of Venus was much hotter than conventional astronomical opinion had held, because of a carbon-dioxide greenhouse effect."
"A repeated claim by Velikovsky is that Venus is cooling off with time. [..] We see that there is not the faintest hint of a decline in temperature with time."[1]

Problem IX. The Craters of Venus

Sagan writes:

"In 1973 an important aspect of the surface of Venus was discovered by Richard Goldstein and associates, using the Goldstone radar observatory of the Jet Propulsion Laboratory. Goldstein et al. found, from radar which penetrates Venus' clouds and is reflected off its surface, that the planet is cratered, and, perhaps, like parts of the moon, is saturation-cratered - that is, so packed with craters that one crater overlaps the other. [..] The clear conclusion from the craters of Venus is, therefore, that Venus has been for billions of years an object exposed to interplanetary collisions - in direct contradiction to the fundamental premise of Velikovsky's hypothesis."[1]

Problem X. The Circularization of the Orbit of Venus and Nongravitational Forces in the Solar System

Sagan writes:

"The idea that Venus could have been converted, in a few thousand years, from an object in a highly eccentric orbit to its present orbit, which is, except for Neptune, the most nearly perfect circular orbit of all the planets, is at odds with what we know about the three-body problem in celestial mechanics."[1]


Sagan writes:

"Of the ten tests of Velikovsky's work described above, there is not one case where his ideas are simultaneously original and consistent with simple physical theory and observation. Moreover, many of the objections - especially those set forth in Problems I, II, III, and X - are objections of high weight, based on the motion and conservation laws of physics."[1]

Criticism of Carl Sagan's paper

Critics of Carl Sagan's paper contend that it contains errors and inaccuracies.

The editors of Pensée write:

"Sagan's ten points -- the phrase "ten plagues" had been suggested by a remark King made following Sagan's talk -- consumed some 37 manuscript pages. A brief enumeration follows. (This enumeration, by extracting the ten basic arguments from the nearly impenetrable mesh of error in which they were embedded, performs a considerable service for Sagan.)"[2]

Physicist and author of the book, The Age of Velikovsky (1976), C. J. Ransom write:

"[Sagan's] paper was presented exceptionally well, and his charisma added to the effectiveness of the presentation. Most of the audience did not know and, because of his captivating delivery, did not care that many of his points were irrelevant, incorrect, or misleading [..] Some of his pre-AAAS meeting comments were drastically revised for the paper given at the meeting. After many fallacious points about his paper were revealed in Pensée, the paper was again corrected. [3]

Author of the book, Carl Sagan and Immanuel Velikovsky (1995), Charles Ginenthal, writes:

"Having read Velikovsky, I also read Sagan's paper; I thereafter discovered that a group of scientists and scholars had written critiques of Sagan's analysis. After reading these criticisms I began a search of the literature and over a period of time I became convinced that Sagan's critique lacked substance. Most surprising was the number of statements made by Sagan that proved to be clearly untrue. Further reading reinforced this discovery of the glaringly unscientific and unscholarly quality of Sagan's paper. What was much worse, was that it was difficult to imagine that even Sagan was unaware of the misrepresentation of evidence presented as scholarly criticism by him and offered to the public."[4]

Velikovsky critic, C. Leroy Ellenberger describes:

"[In] Robert Jastrow's 1,100 word review [of Broca's Brain],[5] .. The chapter on Immanuel Velikovsky", according to Jastrow, "is the most able analysis of this fascinating man's ideas to appear in print". [..] While the Velikovsky chapter "is Professor Sagan at his best", Jastrow finds himself "nagged by doubt"...
"[..] After covering the AAAS Symposium, Jastrow related how "Dr. Velikovsky had his day when he spotted a major scientific boner in Professor Sagan's argument" concerning the odds against the collisions in Worlds in Collision. The "error lay in the assumption that the collisions were independent of one another.... Dr. Velikovsky pointed out that the collisions are not independent; in fact, if two bodies orbiting the sun under the influence of gravity collided once, that encounter enhances the chance of another, a fact well known in celestial mechanics. Professor Sagan's calculations, in effect, ignore the law of gravity. Here Velikovsky was the better astronomer."[6]


  1. a b c d e f g h i j k Carl Sagan "An Analysis of "Worlds in Collision": Introduction", in Donald W. Goldsmith (Editor), Scientists Confront Velikovsky (1977), Cornell University Press, ISBN 0801409616, ISBN-13 978-0801409615. Contents at the SAO/NASA Astrophysics Data System (ADS)
  2. "San Francisco, February 25, 1974", Pensée Vol. 4 No 2: (Spring 1974) "Immanuel Velikovsky Reconsidered VII"
  3. C. J. Ransom, The Age of Velikovsky, 1976, Dell Publishing Co., Inc. ISBN 0-440-50323-X
  4. Charles Ginenthal, Carl Sagan and Immanuel Velikovsky, 1995 New Falcon Publications, 448pages. ISBN 1-56184-075-0. $16.95. Paper bound. Web page (retrieved Oct 28, 2008
  5. Robert Jastrow, "Outer Space and Inner Space," The New York Times Book Review (June 10, 1979), pp. 9, 32.
  6. C. Leroy Ellenberger, "Heretics, Dogmatists and Science's Reception of New Ideas (Part 3)", Kronos Vol. VI No. 2 (Winter 1981)

See also

  • Immanuel Velikovsky, "The Ten Points Of Sagan", Kronos vol.3 No.2, Winter 1977
  • Lewis M. Greenberg, "Sagan's Folly Part 1", Kronos vol.3 No. 2, Winter 1977
  • Ralph E. Juergens, "Sagan's "Ten Plagues", Kronos vol.3 No. 2, Winter 1977
  • C. J. Ransom, "Sagan's Appendices: A Quick Appendectomy", Kronos vol.3 No. 2, Winter 1977
  • Shulamit Kogan, "Sagan vs. Sagan", Kronos vol.6 No.3, Spring 1981
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