This may be to some of our readers a startling question; for most of us have had that star pointed out to us many years; and perhaps those who directed our eyes to it little thought that there would ever be any other pole-star. It is well known that if the northern extremity of the axis of our earth were lengthened until it met the imaginary sphere of the heavens, it would come very near to our present pole-star, hence called Polaris; and if, for any cause, the direction of that axis were materially altered, that star would no longer be a true index of the north. We now propose to show that such a change of the direction of the earth’s axis is continually taking place; and that the terrestrial axis when thus lengthened describes a cone, the apex of which is the centre of the earth; and the circumference of the base of the cone is a circle described amongst the stars. When the axis has described one-half of its course, the angle between the two positions it occupies at the beginning and at the middle of the rotation is about forty-seven degrees. And thus the extremity of the axis will successively come near to other stars than our present pole-star; and in about twelve thousand years it will have as the Polaris the very conspicuous star Vega, or a in the constellation Lyra.

We now proceed to explain the reason of this movement of the earth’s axis. It is well known that the earth is not a perfect sphere, but is flattened at the poles, being what astronomers call an oblate spheroid. Now, the sun’s attraction upon such a spheroidal body is not quite the same as it would be upon a perfect sphere. When the sun is at either equinox—that is, just over the equator—the attraction exercised upon our earth is the same as if that body were spherical; but when the sun is at or near the upper tropic, its action upon the terrestrial matter which bulges at the equator has a tendency to pull that matter towards the ecliptic, and to make the axis of the earth approach to a vertical to the ecliptic. The same influence is at work when the sun is near the lower tropic. And if this influence were not counteracted, the effect would be to cause the ecliptic and equator ultimately to coincide; and our annual succession of seasons would be done away with. But as no such catastrophe is threatening us, and the inclination of the ecliptic to the equator remains about twenty-three and a half degrees, there must be some force which neutralises the above tendency: this is the rotation of the earth on its own axis. No one but a good mathematician could a priori tell the exact effect of these two forces combined. But any one may see how rotation may effect the motion of a body acted on by another force, by observing how a pegtop is kept upright by the rotation, whilst it falls as the rotation ceases. The influence of this rotation to keep a body from falling may be noticed by any one who carefully observes a spinning coin when about to fall. While the coin spins rapidly, its uppermost part appears as a point. As it falls, the point becomes a small circle, increasing as the rotation slackens. But if the coin be very closely watched, when beginning to fall, it will be seen that the small circle is for a moment diminished, showing that the coin had partially recovered its upright position. This recovery is entirely due to the rotation. Similarly, a bicycle is kept from falling by its horizontal motion; and a conical bullet, which has gained a great rapidity of rotation from a rifled barrel, keeps the direction of its axis without deflection to the right or left. And thus we find that the present position of the earth’s axis with respect to the ecliptic is not altered; but the two forces acting upon the earth cause the axis to rotate, as above described, so that the north pole describes a circle in the heavens. But as the period of this rotation is very great, it was not easy to detect such a result, except after a long period of observation. It was discovered thus. The point where the ecliptic and equator cut is called the first point of the constellation Aries, one of the well-known twelve signs of the zodiac. From this point all celestial measurements are made eastwards. Each star of importance has had its distance east of that point—called its right ascension—recorded. In the course of time, the tables of these numbers so recorded appeared to be erroneous; but the error was so regular, and all in one direction, that it was conjectured that the point from which these right ascensions were reckoned had itself shifted its place. And so it proved; and if any one looks at a celestial globe, he will see that Aries no longer occupies the position where the equinox is, but is somewhat to the east, or right, because the point of intersection of the ecliptic and equator has slipped back. But as the sun appears to take a shorter time to come back to the equinox than to arrive at the same stars, which were once close to that point of intersection, this slow retrograde motion is termed the precession of the equinoxes. The distance on the equator caused by this retrograde motion would, if not otherwise modified, be 50·41 annually. But the attraction of the planets on each other produces a very small motion of the equinox in the other direction; and so the resulting precession is about 50·1 annually. If we divide the three hundred and sixty degrees in every circle by the above small quantity, we shall find that the period of the revolution of the earth’s axis is twenty-five thousand eight hundred and sixty-eight years.

Of course the moon has an influence on the extra mass at the earth’s equator, as the sun has, similar in kind, but far less in quantity. This influence would cause the earth’s axis to describe very small cones of the same nature as the large cone above described; and the period of every rotation would be about nineteen years. The effect of this second or lunar influence is to cause the earth’s axis to dip a little towards the equator, and then to resume its position; and this nodding motion is termed nutation, from the Latin word nuto, to nod. Thus the axis of the earth describes a cone not of uniform surface, but as it were fluted, and completes its majestic round in nearly twenty-six thousand years, pointing to a various succession of stars which will in their turns be honored by future astronomers as the pole-stars of their respective generations.—Chambers’s Journal.