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The Moon and the Origin of Life on Earth

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The Moon and the Origin of Life on Earth The Moon and the Origin of Life on Earth Jacques Laskar Laluneet l'origine April 1993∗ del'homme If the Moon did not exist, the orientation of the Earth’s Moon and the Sun. A similar precession of a solid body’s axis axisr JACQUES wouldTASKAR not be stable, and would be subject to large of rotation can easily be observed with an ordinary spinning chaotic variations over the ages. The resulting climatic top. One consequence of this phenomenon is that the Earth’s changesSi la Lune n'existait very likelypas, l'orientation would have de I'axe markedly de rotation disturbedde laTerre the de- rotational axis does not always point toward Polaris, but in- velopmentne seraitpas stable, of organizedet subirait delife. largesvariations chaotiques au cours stead describes a large circle in the celestial sphere. This desAges. Les changementsclimatiques engendrds par cesvariations auraientWe are alors all perturbd familiarfortement with lethe ddveloppement change ofde la seasons vie organisde. due to the fact sometimes disturbs the quibblings of astrologers, since inclination of the equator with respect to the plane of the the precession of the equinoxes has so greatly displaced the Earth’s orbit around the Sun. This inclination of 23◦270, zodiacal calendar from the apparent motion of the Sun that ous sommestous familiersdu lationspossibles de l'obliquit6, et qu'elle Une des cons6quencesde ce ph6no- calledrythme “obliquity” dessaisons, dont la suc- byagit astronomers, donc commeun r6gulateur alsoclimati- givesmdne est rise que toI'axethe de rotation polar de la presently, on the date corresponding to the sign of Aries, the cessionr6sulte de I'inclinaison que de la Terre. Terrene pointe pas toujours vers l'6toile circles,de l'6quateur par inside rapport au of plan which de En day720 avant and J.-C., night Hipparque maypolaire, eachmais d6crit lastun large severalcercle sur la Sun is in the constellation Pisces. I'orbite de la Terre autour du Soleil. d6couvreque la directionde I'axede vo0tec6leste. months.Cette inclinaison The de 23o27', distributionappel6e rotation deof la Terre solar n'est pas heat fixe. En overLa pr6cession the Earth’sdes 6quinoxes sur-consti- The precession of the equinoxes also affects the Earth’s cli- obliquit6par lesastronomes, est aussi la effet,ce dernier ddcrit un c6nedans I'es- tue d'ailleursune objectionimportante facecausede alsoI'existence dependsdes cercles polaires, on thepace,avec obliquity,une p6rioded'environ making26 000 aux itarguties an essentialdes adeptes de I'astrologie. part mate. The Earth’s orbit is in fact not circular, but, as Kepler i I'int6rieurdesquels les jours et lesnuits ans.Ce mouvement,appel6 prdcession carelle a introduitun tel d6calaseentre ofpeuvent ourdurer understanding plusieursmois. Lar6par- des of6quinoxes, terresterialr6sulte de I'existence climate.du le calendrier Calculationset le mouvementapparent car- tition de la quantit6de chaleursolaire bourrelet6quatorial de la Terre,et du du Soleilqu'ir la datecorrespondant au showed in the early 17th century, is instead approximately el- reEued la surfacede la Terred6pend de couple exerc6sur ce bourrelet par la signedu B6lier,le Soleilse trouve main- riedson obliquit6, outqui at estI'un the des6l6ments BureauLune ofet par Longitudes le Soleil.On observe intrds Paristenant dans show la constellation thatdes the Pois- liptic with the Sun residing at one focus. The eccentricity of fondamentauxde la comprdhensiondu facilementce ph6nomdne de pr6cession SONS. Moonclimat.Les calculs stabilizesque nous avons oscillationseffec- de I'axe de rotation in the d'un Earth’scorps solide obliquity,Cettepr6cession atherefore un effet sur le cli- this ellipse (which measures its elongation) is small (0.017), tu6s au Bureau des Longitudes dansun champde pesanteurquand on mat de la Terre.En effet.l'orbite de la actingmontrent que as la Lune a stabilise climaticles oscil- regulatorfait tourner une toupie for sur the une table. Earth.Terren'est pas circulaire. mais approxi- but is enough to measurably change the quantity of solar heat reaching the Earth at perihelion, or point of closest ap- proach to the Sun, as compared to aphelion, its point of far- thest departure from the Sun. At present, the Earth passes through its perihelion January 4th, during the boreal winter. This diminishes seasonal contrasts in the northern hemish- pere, and accentuates them in the southern hemishpere. In 13,000 years, the situation will be reversed and seasonal con- 22 JUIN trasts will be more accentuated in the northern hemisphere. The precession of the equinoxes thus affects the distribution of insolation at a given location on the Earth in the course of 1. L'ALTERNANCE DES SAISONS d6pendde I'obliquit6 e de la la Terre est au point le plus prbs du Soleil (p6rih6lie), pendant l'6t6 Terre (a) et de la pr6cessiondes 6quinoxes (D). En 616,la quantit6 de de I'h6misphbre Nord ou pendant I'hiver, comme c'est le cas actuel- chaleur solaire regue dans I'h6misphbreNord est plus importante lement. Il en r6sulte une augmentation, ou une diminution du a year. In fact, it appears possible to trace more significant Figureque dansI'h6misphbre 1: TheSud. Selon changela valeur de I'angle of de seasons pr6cession, contraste depends des saisons. on the Earth’s obliq- climatic changes to variations in the Earth’s eccentricity and uity14 (a) and on the precession of the equinoxes (b). InO poun sum-LA sc ENCE obliquity. mer, the amount of solar heat received in the northern hemi- sphere is greater than that received in the southern hemi- sphere. Depending on the angle of precession, the Earth may be at its point nearest the Sun (perihelion) during the north- The Astronomical Theory of Climates ern hemisphere’s summer, or during its winter, as is currently In Kepler’s view, the Earth’s orbit was an immutable el- the case. The hemispheres thus experience a corresponding lipse. Newton challenged this view by demonstrating that accentuation or dimunition of seasonal contrasts. the masses of the other planets perturbed the Earth’s orbit, so that it is only an ellipse to first approximation: neither its In 120 BC, Hipparcus discovered that the direction of the eccentricity nor its obliquity are fixed. LeVerrier (famed for Earth’s rotational axis is not fixed with respect to the stars. his discovery in 1846 of the planet Neptune based on pertur- In fact, it traces out a cone in space roughly once every 26,000 bations of Uranus’ orbit) was the first to calculate possible years. This so-called precession of the equinoxes is the result long-term (or “secular”) variations in the Earth’s eccentricity. of the torque exerted on the Earth’s equatorial bulge by the In doing so, he took up calculations of the Earth’s orbital mo- tion begun by Laplace shortly before the French Revolution. ∗This text is a close translation (by H.S. Dumas) of the original paper La Lune et l’origine de l’homme that was first published in the french It was LeVerrier’s Earth-orbit calculations that, in 1941, led version of Scientific American, Pour la Science, 186, pp 34 - 41, April the Yugoslavian astronomer Milutin Milankovich to hypoth- 1993, shortly after the publication of the associated research papers in esize that the ice ages were the result of high-lattitude vari- Nature. This paper has since been reprinted in several foreign editions ations in terrestrial insolation induced by secular variations of Scientific American (Italian, Arabic, Japanese, German, Polish), and in Portugese in Cincia Hoje. Up to now (september 2009), It was not in the Earth’s orbit and axial orientation. His theory did not available in English. win immediate acceptance, as the variations in insolation did 1 not seem adequate to account for glaciation. But the same as it melts ice accumulated over the winter and prevents the theory has gained wider acceptance over the last two decades. ice caps from extending their reach. Weak variations in the Measurements carried out by John Imbrie and coworkers of Earth’s obliquity are therefore a determining factor in regu- the relative concentration of the oxygen isotopes O18 and O16 lating the relatively moderate climate enjoyed by the Earth present in the carbonates of marine sedimentary layers can over the last several million years, and in allowing the ap- be related to the past thickness of the polar ice caps. From pearance of organized life as we understand it. The ice ages this it is possible to estimate mean ocean temperatures in the constitute significant climatic changes, but were not so se- distant past. Indeed, it provides some record of the Earth’s vere as to permanently change the conditions for life on the past climate up to more than three million years ago. Though Earth’s surface. mativementelliptique, comme l'a mon- diminue le contrastedes saisonsdans La theorieastronomique muchtrd Kepleren less 1609,le precise, Soleiloccupant un geologicalI'h6misphdre records reflect climatic condi- Nord, alors qu'elle despal6oclimats desfoyers de l'ellipse.L'excentricit6 de I'accentuedans I'h6misphdre Sud. Dans I'ellipse(qui mesureson aplatissement) 13 tions as far back as 200000million ans,I'effet sera years.contraire, et les Moreover,Pour Kepler, la improved Terre d6crivaitune estfaible (0,017),mais suffit pour chan- contrastesdes saisons seront plus accen- ellipseimmuable. Newton a boulevers6 ger la quantit6 de chaleur reEued la models of climatic responsetu6sdans I'h6misphdre to variationsNord. La pr6ces- incet the 6quilibre Earth’sen montrantque orbit la masse Variations in the Earth’s Obliquity surfacede la Terre, entre le p6rih6lie, sion des 6quinoxesmodifie donc la des autresplanbtes du Sys[dmesolaire positionoD la Terre est le plus prdsdu r6partition show that the effects of changesde I'insolation inen insolation un lieu de perturbait mayl'orbite be de laampli- Terre,qui n,est Soleil,et I'aph6lieoi elleest d son6loi- la Terreau coursde I'ann6e.En fait,des donc une ellipse qu'en premibre gnementmaximal.
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