this line, altering its position in space; but amidst all these changes, the line of the apsides knows no change. Its length remains the same notwithstanding all the perturbations of the planets, and all the changes to which the rest of the orbit may be subjected. The eccentricity of the earth's orbit is now slowly changing, diminishing at the rate of about 11" in a century, on account of the mutual gravitation of the planets. It will continue to diminish for centuries, and it is somewhat curious that this dimunition is indicated by the movements of the moon, for it produces, as we have remarked before, no effect upon the periodic time of a revolution of the earth. It is now pretty well determined that the motion of the moon is continually accelerated. Upon comparing observations made at distant intervals, it is found that the Chaldean and Alexandrian observations give a longer period, than the Alexandrian and Arabian of the eighth century; and a comparison of the Arabian and modern observations, gives a still shorter period. These variations are all periodical, and are compensated in opposite points of every period, the mean distanand mean periods, will always remain the same. "Cold, we think, must be the heart that is not affected by this mark of beneficent wisdom in the contriver of the magnificent fabric." ces, INCLINATION OF THE EARTH'S AXIS. 105 CHAPTER IX. The Seasons. "These, as they change, Almighty Father, these Thompson. Is full of thee." THROUGHOUT the preceding chapter, we have spoken of the plane of the orbit of the earth as inclined to the plane of the equator. As a necessary result of this, the axis of the earth points, at all times, towards the same point of the heavens, with the exception of the slight motion caused by the precession of the equinoxes. This is a fact, with which we are all familiar, for we know the pole star is the star towards which the axis of the earth points throughout the year. Thus, in the following diagram, let B A B C D, represent the earth in four different positions in its orbit. Now, if the axis of the earth was perpendicular to the plane of the orbit, as shown in the diagram, then the plane of the equator, a, b, would correspond with the plane of the orbit, but, as the plane of the equator manifestly does not correspond with the plane of the ecliptic, the axis of the earth must be inclined to the plane of its orbit, as in the next diagram, and this inclination it maintains throughout its entire revolution around the sun, its axis always pointing towards the polar star. Of course, in our diagram, this does not appear to be strictly the case, as we cannot represent the star removed to a sufficient distance. The inclination of the axis of the earth to the plane of the sun's apparent path, is the cause of all the variety of the seasons; of the differing lengths of the nights and days; and the daily changes of the sun's declination. If the earth's axis had been placed perpendicular to the plane of the ecliptic, then, at all seasons of the year, the days and nights would have been equal all over the world, for, as will be seen on reference to the figure on page 105, the sun would shine from pole to pole, and as it would illuminate but half the globe at once, a spectator any where on its surface, would be just as long in passing through the unilluminated as the illuminated portion. Thus, a spectator at A, on the parallel of latitude B C, would be just as long in moving through the half of the unilluminated portion of the earth B A, as through half the illuminated A C. This is the case at the equinoxes. When the sun is in either of those points of his orbit which crosses the equator, then it illuminates the globe from pole to pole, i. e. when he is in either of the points 0, or XII, see the figure on page 91. We have already shown that the apparent diurnal path of the sun through the heavens due to a rotation of the earth upon its axis, is in what is called a diurnal DECLINATION OF THE SUN. 107 circle, or a circle parallel to the equator; hence, at the time of the equinoxes, the sun will, apparently, in his diurnal circle in the heavens, move in the equator; and as the successive meridians come under, it will appear directly over head, or vertical, at noon, to a person any where on the equator. As the globe turns around, the sun now passes directly over the islands of Sumatra and Borneo, and the islands in the midst of the Pacific ocean; also over the northern part of South America, the middle of Africa, and the Indian ocean. We will suppose it is the time of the autumnal equinox, when the sun enters Libra, or is in the position marked XII, in the figure on page 91. A month after, the sun has moved to the position XIV. And as the earth turns around on its axis, it appears no longer vertical, or over head, at the equator, at noon, but to those places situated on the parallel eƒ, over the islands of New Guinea and Java; the middle of South Africa; the top of the Brazils; and, perhaps, the Society, and the Friendly islands. As the sun moves still farther on in its orbit, to the position VI, it appears now vertical, or over head, at noon, to an observer on the parallel g h, and as this is the greatest distance from the equator, and therefore farthest south where it can be vertical, at noon, and, as after this, it again approaches the equator, this point is called the solstitial point, i. e. the point where, having reached its greatest southern declination, the sun, apparently, for a few days, remains still, or at precisely the same altitude at noon, for a few days, and then begins to return; this limiting parallel on the earth is called the tropic of Capricorn, for it is now January, the time of the winter solstice, when the sun enters the sign Capricorn. The earth is now illuminated by the E M H B sun, as shown in this diagram. The sun being vertical at noon, to all places on the parallel C D, it will be observed that it is now summer in the southern hemisphere, and that there, the days are now longer than the nights, the illuminated portion C G, being greater than the shaded portion G D. At the equator, however, the days and nights are still equal, but in the northern hemisphere it is mid-winter, and the days are shorter than the nights, as the arc E H, is shorter than the arc H F, the former representing half the day, the latter half the night. Here, then, is the explanation of the short days in winter, and the long nights, and it will also be seen that, to an observer any where in the northern hemisphere, the sun will come to the meridian very low down. It will also be noticed, that the shadow of the unilluminated portion of the earth, falls entirely beyond, or without the antarctic circle I K, and includes the arctic circle L M. To an observer, therefore, within the southern polar circle, the sun now never sets. Three months before, when the earth and sun were in the positions shown in the figure, page 106, the sun rose a short way above the horizon, within each circle, but each succeeding day he sank lower and lower, to those within the arctic or northern polar circle, and rose higher and higher to those within the antarctic circle, so that now, begins the long day of the latter, and the long night of the former. It would be a curious sight to an inhabitant of the more temperate zones, to see the sun thus gradually mounting above the horizon, moving completely around without setting, and visible during the whole day, for nearly six months. Although darkness reigns at the other pole, so far as the direct rays of the sun are concerned, yet the long night is enlivened by the bright moon light, which reflected from a thousand hills of snow, sheds a bright light around, and the planets and stars in that cold region, where no mists ever obscure the sky, twinkle in the clear firmament like diamonds. The bright corruscations of the Aurora, with changing and fanciful lights, are there seen in their greatest perfection, and cheer, with their varying forms, the hunters who penetrate within the icy circle; and here nature is seen in some of her grandest forms. Huge mountains of ice, formed by the winters of centuries, rear their Alpine summits to the sky, and life in singular forms, sport on its |