W. & A. K. Johnston, Edinbr. and London.
DIAGRAM REPRESENTING THE VARIATIONS IN THE ECCENTRICITY OF THE EARTH’S ORBIT FOR THREE MILLION OF YEARS BEFORE 1800 A.D. ONE MILLION OF YEARS AFTER IT.
The Ordinates are joined by straight lines where the values, at intervals of 10,000 years, between them have not been determined.
In this Table there are seven periods when the earth’s orbit becomes nearly circular, four in the past and three in the future.
The Table shows also four or five subordinate periods of high eccentricity, the principal one occurring 200,000 years ago.
The variations of eccentricity during the four millions of years, are represented to the eye diagrammatically in [Plate IV.]
In order to determine with more accuracy the condition of the earth’s orbit during the three periods of great eccentricity included in Table I., I computed the values for periods of ten thousand years apart, and the results are embodied in Tables II., III., and IV.
There are still eminent astronomers and physicists who are of opinion that the climate of the globe never could have been seriously affected by changes in the eccentricity of its orbit. This opinion results, no doubt, from viewing the question as a purely astronomical one. Viewed from an astronomical standpoint, as has been already remarked, there is actually nothing from which any one could reasonably conclude with certainty whether a change of eccentricity would seriously affect climate or not. By means of astronomy we ascertain the extent of the eccentricity at any given period, how much the winter may exceed the summer in length (or the reverse), how much the sun’s heat is increased or decreased by a decrease or an increase of distance, and so forth; but we obtain no information whatever regarding how these will actually affect climate. This, as we have already seen, must be determined wholly from physical considerations, and it is an exceedingly complicated problem. An astronomer, unless he has given special attention to the physics of the question, is just as apt to come to a wrong conclusion as any one else. The question involves certain astronomical elements; but when these are determined everything then connected with the matter is purely physical. Nearly all the astronomical elements of the question are comprehended in the accompanying Tables.
TABLE I.
The Eccentricity and Longitude of the Perihelion of the Earth’s Orbit for 3,000,000 Years in the Past and 1,000,000 Years in the Future, computed for Intervals of 50,000 Years.
| PAST TIME. | ||
|---|---|---|
| Number of years before epoch 1800. | Eccentricity. | Longitude of perihelion. |
° ′ | ||
−3,000,000 | 0·0365 | 39 30 |
−2,950,000 | 0·0170 | 210 39 |
−2,900,000 | 0·0442 | 200 52 |
−2,850,000 | 0·0416 | 0 18 |
−2,800,000 | 0·0352 | 339 14 |
−2,750,000 | 0·0326 | 161 22 |
−2,700,000 | 0·0330 | 65 37 |
−2,650,000 | 0·0053 | 318 40 |
−2,600,000 | 0·0660 | 190 4 |
−2,550,000 | 0·0167 | 298 34 |
−2,500,000 | 0·0721 | 338 36 |
−2,450,000 | 0·0252 | 109 33 |
−2,400,000 | 0·0415 | 116 40 |
−2,350,000 | 0·0281 | 308 23 |
−2,300,000 | 0·0238 | 195 25 |
−2,250,000 | 0·0328 | 141 18 |
−2,200,000 | 0·0352 | 307 6 |
−2,150,000 | 0·0183 | 307 5 |
−2,100,000 | 0·0304 | 98 40 |
−2,050,000 | 0·0170 | 334 46 |
−2,000,000 | 0·0138 | 324 4 |
−1,950,000 | 0·0427 | 120 32 |
−1,900,000 | 0·0336 | 188 31 |
−1,850,000 | 0·0503 | 272 14 |
−1,800,000 | 0·0334 | 354 52 |
−1,750,000 | 0·0350 | 65 25 |
−1,700,000 | 0·0085 | 95 13 |
−1,650,000 | 0·0035 | 168 23 |
−1,600,000 | 0·0305 | 158 42 |
−1,550,000 | 0·0239 | 225 57 |
−1,500,000 | 0·0430 | 303 29 |
−1,450,000 | 0·0195 | 57 11 |
−1,400,000 | 0·0315 | 97 35 |
−1,350,000 | 0·0322 | 293 38 |
−1,300,000 | 0·0022 | 0 48 |
−1,250,000 | 0·0475 | 105 50 |
−1,200,000 | 0·0289 | 239 34 |
−1,150,000 | 0·0473 | 250 27 |
−1,100,000 | 0·0311 | 55 24 |
−1,050,000 | 0·0326 | 4 8 |
−1,000,000 | 0·0151 | 248 22 |
− 950,000 | 0·0517 | 97 51 |
− 900,000 | 0·0102 | 135 2 |
− 850,000 | 0·0747 | 239 28 |
− 800,000 | 0·0132 | 343 49 |
− 750,000 | 0·0575 | 27 18 |
− 700,000 | 0·0220 | 208 13 |
− 650,000 | 0·0226 | 141 29 |
− 600,000 | 0·0417 | 32 34 |
− 550,000 | 0·0166 | 251 50 |
− 500,000 | 0·0388 | 193 56 |
− 450,000 | 0·0308 | 356 52 |
− 400,000 | 0·0170 | 290 7 |
− 350,000 | 0·0195 | 182 50 |
− 300,000 | 0·0424 | 23 29 |
− 250,000 | 0·0258 | 59 39 |
− 200,000 | 0·0569 | 168 18 |
− 150,000 | 0·0332 | 242 56 |
− 100,000 | 0·0473 | 316 18 |
− 50,000 | 0·0131 | 50 14 |
| FUTURE TIME. | ||
| Number of years after epoch 1800. | Eccentricity. | Longitude of perihelion. |
° ′ | ||
a.d 1800 | 0·0168 | 99 30 |
+ 50,000 | 0·0173 | 38 12 |
+ 100,000 | 0·0191 | 114 50 |
+ 150,000 | 0·0353 | 201 57 |
+ 200,000 | 0·0246 | 279 41 |
+ 250,000 | 0·0286 | 350 54 |
+ 300,000 | 0·0158 | 172 29 |
+ 350,000 | 0·0098 | 201 40 |
+ 400,000 | 0·0429 | 6 9 |
+ 450,000 | 0·0231 | 98 37 |
+ 500,000 | 0·0534 | 157 26 |
+ 550,000 | 0·0259 | 287 31 |
+ 600,000 | 0·0395 | 285 43 |
+ 650,000 | 0·0169 | 144 3 |
+ 700,000 | 0·0357 | 17 12 |
+ 750,000 | 0·0195 | 0 53 |
+ 800,000 | 0·0639 | 140 38 |
+ 850,000 | 0·0144 | 176 41 |
+ 900,000 | 0·0659 | 291 16 |
+ 950,000 | 0·0086 | 115 13 |
+1,000,000 | 0·0528 | 57 31 |