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 α
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._