Superior limits of eccentricity as determined by Lagrange, Leverrier,and Mr. Stockwell:—
| By Lagrange. | By Leverrier. | By Mr. Stockwell. | |
|---|---|---|---|
| Mercury | 0·22208 | 0·225646 | 0·2317185 |
| Venus | 0·08271 | 0·086716 | 0·0706329 |
| Earth | 0·07641 | 0·077747 | 0·0693888 |
| Mars | 0·14726 | 0·142243 | 0·139655 |
| Jupiter | 0·06036 | 0·061548 | 0·0608274 |
| Saturn | 0·08408 | 0·084919 | 0·0843289 |
| Uranus | — | 0·064666 | 0·0779652 |
| Neptune | — | — | 0·0145066 |
[J. C.]
[316] “Mém. de l’Acad. royale des Sciences.” 1827. Tom. vii., p. 598.
[317] Absolute zero is now considered to be only 493° Fah. below the freezing-point, and Herschel himself has lately determined 271° below the freezing-point to be the temperature of space. Consequently, a decrease, or an increase of one per cent. in the mean annual amount of radiation would not produce anything like the effect which is here supposed. But the mean annual amount of heat received cannot vary much more than one-tenth part of one per cent. In short, the effect of eccentricity on the mean annual supply of heat received from the sun, in so far as geological climate is concerned, may be practically disregarded.—[J. C.]
[318] “Principles of Geology,” p. 110. “Mr. Lyell, however, in stating the actual excess of eight days in the duration of the sun’s presence in the northern hemisphere over that in the southern as productive of an excess of light and heat annually received by the one over the other hemisphere, appears to have misconceived the effect of elliptic motion in the passage here cited, since it is demonstrable that whatever be the ellipticity of the earth’s orbit the two hemispheres must receive equal absolute quantities of light and heat per annum, the proximity of the sun in perigee exactly compensating the effect of its swifter motion. This follows from a very simple theorem, which may be thus stated: ‘The amount of heat received by the earth from the sun while describing any part of its orbit is proportional to the angle described round the sun’s centre,’ so that if the orbit be divided into two portions by a line drawn in any direction through the sun’s centre, the heats received in describing the two unequal segments of the ellipse so produced will be equal.”
[319] When the eccentricity of the earth’s orbit is at its superior limit, the absolute quantity of heat received by the globe during one year will be increased by only 1/300th part; an amount which could produce no sensible influence on climate.—[J. C.]
[320] Sir Charles has recently, to a certain extent, adopted the views advocated in the present volume, viz., that the cold of the glacial epoch was brought about not by a decrease, but by an increase of eccentricity. (See vol. i. of “Principles,” tenth and eleventh editions.) The decrease in the mean annual quantity of heat received from the sun, resulting from the decrease in the eccentricity of the earth’s orbit—the astronomical cause to which he here refers—could have produced no sensible effect on climate.—[J. C.]
[321] It is singular that both Arago and Humboldt should appear to have been unaware of the researches of Lagrange on this subject.
[322] “Révolutions de la Mer,” p. 37. Second Edition.