There is some reason, also, to suspect, that there is a difference of temperature on opposite sides of the sun. As the synodical rotation is nearly identical with the siderent period of the moon, this would require about forty-four years to run its course, so as to bring the phenomena to exact coincidence again. Since these observations were made, it is understood that Sig. Secchi has determined that the equatorial regions of the sun are hotter than his polar regions. It may be owing to this fact, that we have inferred a necessity for a change, whose period is a multiple of the sun’s synodical rotation, but it is worthy of examination by those who possess the necessary conveniences.
Another period which must influence the character of different years, depends on the conjunction of the perigee of the lunar orbit with the node. Taking the mean direct motion of the moon’s perigee, and the mean retrograde motion of the node, we find that it takes six years and one day nearly from conjunction to conjunction. Now, from the principles laid down, it follows, that when the perigee of the orbit is due north, and the ascending node in Aries, that the vortices of the earth will attain their greatest north latitude; and when these conditions are reversed, the vortices will reach their highest limit in the lowest latitude. This will materially affect the temperature of the polar regions. In the following table, we have calculated the times of the conjunctions of the apogee and pole of the orbit, taking the mean motions. It may be convenient to refer to by-and-bye, remembering that when the conjunction takes place due south, the vortices reach the highest, but when due north, the vortices in the northern hemisphere have their lowest upper limit:
| Year. | Month and Day. | Longitude. | |
|---|---|---|---|
| 1804, | April | 18th, | 220° |
| 1810, | " | 17th, | 104° |
| 1816, | " | 16th, | 348° |
| 1822, | " | 15th, | 232° |
| 1828, | " | 14th, | 116° |
| 1834, | " | 12th, | 360° |
| 1840, | " | 11th, | 244° |
| 1846, | " | 10th, | 128° |
| 1852, | " | 9th, | 12° |
| 1858, | " | 8th, | 255° |
| 1864, | " | 7th, | 139° |
| 1870, | " | 6th, | 23° |
| 1876, | " | 5th, | 267° |
By this we see that the vortices have never attained their highest limit during the present century, but that in 1858 their range will be in a tolerable high latitude, and still higher in 1876—neglecting the eccentricity of the orbit.
A very potent influence is also due to the heliocentric longitude of the sun, in determining the character of any given year. Let us explain:
The moon’s inertia forces the earth from the mechanical centre of the terral system, but is never able to force her clear from the central axis. With the sun it is different. He possesses many satellites (planets). Jupiter alone, from his great mass and distance, is able to displace the whole body of the sun. If other planets conspire at the same side, the centre of the sun may be displaced a million of miles from the mechanical centre of the solar system. Considering this centre, therefore, as the centre of an imaginary sun, from which heliocentric longitudes are reckoned, the longitude of the real sun will vary with the positions of the great planets of the system. Now, although this systematic longitude will not be exactly similar to the heliocentric longitude reckoned from the sun’s centre, yet, for the purposes intended, it will correspond sufficiently, and we shall speak of the longitude of the sun as if we reckoned heliocentric longitudes from the mechanical centre of the system. When we come to consider the solar spots, we shall enter into this more fully. In the following diagram we shall be able to perceive a cause for variation of seasons in a given year, as well as for the general character of that year.
Let S represent the centre of the sun, and the circle a vertical section of the sun, cutting; through the centre,—SJ being in the equatorial plane of the vortex, of which ZZ′ represents the axis. As the ether descends the poles or axis at Z, it is met by the current down the opposite pole, and is thence deflected in radii along the equatorial plane to J. But on the side S, the ether is opposed by the body of the sun; its direction is consequently changed, and cross currents are produced, assuming it as a principle, that the ethereal fluid is permeable by other currents of similar matter, and that it tends always to move in right lines. This granted, it is evident that, in passing the sun, the quick moving ether forms a conical shell, (the sun being at the apex,) so that the strongest current of ether is in this conical shell, or at the surface of this conical space. As the plane of the ecliptic is not much inclined to the sun’s equator, and this last probably not much inclined to the plane of the vortex, should the earth have the same heliocentric longitude at the time, (or nearly the same,) she would be in an eddy, as respects the radial stream, and be protected from its full force by the body of the sun.
Now, the ether comes down the axis with the temperature of space, and may possibly derive a little additional temperature in passing over the body of the sun; so that in this position the earth is protected from the chilling influence of the radial stream, by being protected by the body of the sun. And although, from the immense velocity of the ether, it cannot derive much additional temperature, there may still be an appreciable difference, due to this cause.