THE PLANETS. 129 



aphelion six months afterward, upon the first day of July, it 

 may happen, on account of the advance (turm7ig) of the 

 major axis of the Earth's orbit, that the minimum may oc- 

 cur m summer and the maximum in winter, so that in Jan- 

 uary the Earth would be farther from the Sun than in the 

 summer by about 2,800,000 geographical miles {i. e., about 

 3'^th of the mean distance of the Earth from the Sun). It 

 might, at the first glance, be supposed that the occurrence 

 of the perihelion at an opposite time of the year (instead of 

 the winter, as is now the case, in summer) must necessarily 

 produce great climatic variations ; but, on the above suppo- 

 sition, the Sun will no longer remain seven days longer in 

 the northern hemisphere ; no longer, as is now the case, 

 traverse that part of the ecliptic from the autumnal equinox 

 to the vernal equinox, in a space of time which is one week 

 shorter than that in which it traverses the other half of its 

 orbit from the vernal to the autumnal equinox. The differ- 

 ence of temperature which is considered as the consequence 

 to be apprehended from the turning of the major axis (and 

 we refer here merely to the astronomical climates, excluding 

 all considerations as to the relations of the solid and liquid 

 portion of ihe many-formed surface of the Earth) will, on the 

 whole, disappear,^ principally from the circumstance that 

 the point of our planet's orbit in which it is nearest to the 

 Sun is at the same time always that over which it passes 

 with the greatest velocity. The reassuring solution of this 

 problem is to a certain extent contained in the beautiful law 

 first pointed out by Lambert,! according to which the quan- 

 tity of heat which the Earth receives from the Sun in each 

 part of the year is proportional to the angle which the radius 

 vector of the Sun describes during the same period. 



* Arago, in the Annuaire for 1834, p. 199. 



t " II s'ensuit (du theoreme dA k Lambert) que la quantite de cha 

 leur envoyee par le Soleil k la Terre est la merae en allant de r6qui 

 noxe du prin temps k I'equinoxe d'automne qu'en revenant de celui-ci au 

 premier. Le temps plus long que le Soleil emploie dans le premier 

 trajet, est exactement compense par son eloignementaussiplus grand; 

 et les qnantites de chaleur qu'il envoie k la Terre, sont les m^mes pen 

 dant qu'il se trouv»dans I'un ou I'autre hemisphere, boreal ou austral." 

 — Poisson, Sur la StabiliiS du Systeme Planitaire, Connaissance des 

 2'emps for 1836, p. 54. " It follows, from the theorem of Lambert, that 

 tl:3 quantity of heat which is conveyed by the Sun to the Earth is the 

 same during the passage from the vernal to the autumnal equinox as in 

 returning from the latter to the former. The much longer time which 

 the Sun takes in the first part of its course is exactly compensated by 

 its proportionately greater distance, and the quantities of heat which 



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