Practical Talks by an Astronomer

by Harold Jacoby

The pole of the frozen North is not the only pole sought with determined effort by more than one generation of scientific men. Up in the sky astronomers have another pole which they are following up just as vigorously as ever Arctic explorer struggled toward the difficult goal of his terrestrial journeying. The celestial pole is, indeed, a fundamentally important thing in astronomical science, and the determination of its exact position upon the sky has always engaged the closest attention of astronomers. Quite recently new methods of research have been brought to bear, promising a degree of success not hitherto attained in the astronomers' pursuit of their pole.

In the first place, we must explain what is meant by the celestial pole. We have already mentioned the poles of the earth ([p. 136]). Our planet turns once daily upon an axis passing through its centre, and it is this rotation that causes all the so-called diurnal phenomena of the heavens. Rising and setting of sun, moon, and stars are simply results of this turning of the earth. Heavenly bodies do not really rise; it is merely the man on the earth who is turned round on an axis until he is brought into a position from which he can see them. The terrestrial poles are those two points on the earth's surface where it is pierced by the rotation axis of the planet. Now we can, if we choose, imagine this axis lengthened out indefinitely, further and further, until at last it reaches the great round vault of the sky. Here it will again pierce out two polar points; and these are the celestial poles.

The whole thing is thus quite easy to understand. On the sky the poles are marked by the prolongation of the earth's axis, just as on the earth the poles are marked by the axis itself. And this explains at once why the stars seem nightly to revolve about the pole. If the observer is being turned round the earth's axis, of course it will appear to him as if the stars were rotating around the same axis in the opposite direction, just as houses and fields seem to fly past a person sitting in a railway train, unless he stops to remember that it is really himself who is in motion, and not the trees and houses.

The existence of such a centre of daily motions among the stars once recognized, it becomes of interest to ascertain whether the centre itself always retains precisely the same position in the sky. It was discovered as early as the time of Hipparchus ([p. 39]) that such is not the case, and that the celestial pole is subject to a slow motion among the stars on the sky. If a given star were to-day situated exactly at the pole, it would no longer be there after the lapse of a year's time; for the pole would have moved away from it.

This motion of the pole is called precession. It means that certain forces are continually at work, compelling the earth's axis to change its position, so that the prolongation of that axis must pierce the sky at a point which moves as time goes on. These forces are produced by the gravitational attractions of the sun, moon, and planets upon the matter composing our earth. If the earth were perfectly spherical in shape, the attractions of the other heavenly bodies would not affect the direction of the earth's rotation-axis in the least. But the earth is not quite globular in form; it is flattened a little at the poles and bulges out somewhat at the equator. (See [p. 135.])

This protuberant matter near the equator gives the other bodies in the solar system an opportunity to disturb the earth's rotation. The general effect of all these attractions is to make the celestial pole move upon the sky in a circle having a radius of about 23½ degrees; and it requires 25,800 years to complete a circuit of this precessional cycle. One of the most striking consequences of this motion will be the change of the polar star. Just at present the bright star Polaris in the constellation of the Little Bear is very close to the pole. But after the lapse of sufficient ages the first-magnitude star Vega of the constellation Lyra will in its turn become Guardian of the Pole.

It must not be supposed, however, that the motion of the pole proceeds quite uniformly, and in an exact circle; the varying positions of the heavenly bodies whose attractions cause the phenomena in question are such as to produce appreciable divergencies from exact circular motion. Sometimes the pole deviates a little to one side of the precessional circle, and sometimes it deviates on the other side. The final result is a sort of wavy line, half on one side and half on the other of an average circular curve. It takes only nineteen years to complete one of these little waves of polar motion, so that in the whole precessional cycle of 25,800 years there are about 1,400 indentations. This disturbance of the polar motion is called by astronomers nutation.

The first step in a study of polar motion is to devise a method of finding just where the pole is on any given date. If the astronomer can ascertain by observational processes just where the pole is among the stars at any moment, and can repeat his observations year after year and generation after generation, he will possess in time a complete chart of a small portion at least of the celestial pole's vast orbit. From this he can obtain necessary data for a study of the mathematical theory of attractions, and thus, perhaps, arrive at an explanation of the fundamental laws governing the universe in which we live.

The instrument which has been used most extensively for the study of these problems is the transit ([p. 118]) or the "meridian circle." This latter consists of a telescope firmly attached to a metallic axis about which it can turn. The axis itself rests on massive stone supports, and is so placed that it points as nearly as possible in an east-and-west direction. Consequently, when the telescope is turned about its axis, it will trace out on the sky a great circle (the meridian) which passes through the north and south points of the horizon and the point directly overhead. The instrument has also a metallic circle very firmly fastened to the telescope and its axis. Let into the surface of this circle is a silver disk upon which are engraved a series of lines or graduations by means of which it is possible to measure angles.

Observers with the meridian circle begin by noting the exact instant when any given star passes the centre of the field of view of the telescope. This centre is marked with a cross made by fastening into the focus some pieces of ordinary spider's web, which give a well-marked, delicate set of lines, even under the magnifying power of the telescope's eye-piece. In addition to thus noting the time when the star crosses the field of the telescope, the astronomer can measure by means of the circle, how high up it was in the sky at the instant when it was thus observed.