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No. 87: May-Jun 1993

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The earth: a doubly charmed planet

In SF#85, we learned that the evolution of advanced life forms on earth may have depended upon the protective influences of Jupiter and Saturn. These two giant planets can gravitationally deflect potentially devastating asteroids and comets away from the earth.

It seems now that we are doubly lucky! Computer runs demonstrate that the presence of our large moon has stabilized the earth's spin axis down the eons. Presently, the earth's spin axis makes an angle of 23.5 with the plane of the earth's orbit (its "obliquity"). The well-known result is our yearly procession of seasons. Without the steadying effect of the moon, however, the earth's obliquity would probably have swung chaotically over much larger values. Such extreme changes would have been inimical to the development of life, particularly advanced life.

As a case in point, the polar axis of Mars, with only two tiny moons to dampen its spin excursions, seems to have gone through many wild swings, as indicated in the figure. What deadly climatic changes must have wracked our sister planet!

(Touma, Jihad, and Wisdom, Jack; "The Chaotic Obliquity of Mars," Science, 259: 1294, 1993. Also: Laskar, J., and Robutel, P.; "The Chaotic Obliquity of the Planets," Nature, 361:608, 1993.)

Comment. The successful evolution of higher life on earth (a presumption!) therefore seems to have depended upon the gravitational shields of Jupiter and Saturn as well as the presence of our unusually large satellite. How likely is this combination of planets and satellites in the rest of the universe? Of course, life-as-we-know-it also requires just the right kind of central star and a planet with good air and water. Perhaps lifeas-we-do-not-know-it is more likely!

From Science Frontiers #87, MAY-JUN 1993. 1993-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987