Everyday Objects; Or, Picturesque Aspects of Natural History.

by W. H. Davenport Adams

Bradley resumed the idea of his illustrious compatriot. He recognised in the northward movement of the stars the effect of a similar rotation, but one which took much longer in its accomplishment. By doubling the interval of nine years, to the term of which he had seen the movement become stationary, he obtained a period nearly approaching that which the moon employs in returning to the same nodes. This coincidence flashed upon him like a ray of light.

We must here remind the reader,—who, we hope, is not weary of our scientific or semi-scientific disquisition,—that the lunar nodes,—i.e., the points of the Ecliptic through which the moon passes when it proceeds from south to north (the ascending node), and from north to south (the descending node),—are the analogues of the solar equinoxes; the equatorial points through which the sun passes on its course from south to north (the spring equinox), and in returning from north to south (the autumn equinox),—points of intersection whose retrogradation constitutes, as we have seen, the precession of the equinoxes. Well, the moon's nodes retrograde in a similar manner by a movement directed from east to west; only it is a much slower movement. While the equinoxes are displaced but fifty seconds (50") in a year, the lunar nodes, during the same period, and in the same direction, move over a space of 19° 20' 29"; so that, in less than nineteen years, they have made the complete circuit of the heavens, to return to exactly the same point, after traversing 360°.

Thus, then, we have explained the data on which Bradley rested his prediction. It was confirmed by Le Monnier, who observed, in fact, that the star γ in Draco, and the neighbouring stars, observed by Bradley from 1727 to 1736, moving from south to north, occupied the same period of time, from 1736 to 1745, in accomplishing an equal excursion in a contrary direction, from north to south. These observations enabled him to fix approximatively the quantity of the nutation.

To sum up; it was recognised that the angle made by the axis of the terrestrial poles with the axis of the poles of the Ecliptic, far from remaining constantly equal to itself (the amount was 23° 27' 30" in the middle of the present century), varies by 0".48 yearly, and that this angle itself experiences a variation whose mean value is 48" in a century. It sometimes exceeds this mean value, and sometimes falls below it, by an amount which rises to nearly 9".65. Thus, while describing, in an interval of 25,000 to 26,000 years, its curve around the poles of the Ecliptic, the earth's axis describes, from east to west, a small ellipsis in the space of about eighteen and two-third years, and imperceptibly changes, moreover, its angle of inclination.

But, in fine, what is the true cause of all these movements?

Were the earth a perfect sphere, were all its radii of equal length, the effect of the universal ponderation would make itself felt as if all the material molecules were concentrated at a single point—the centre; and, apart from this ponderation, which exercises itself in the direct ratio of the masses, and in the inverse ratio of the square of the distances, nothing exists which would sway our globe in one direction rather than in another,—no precession of the equinoxes would take place, the plane of the Ecliptic would invariably coincide with the plane of the Equator, and an eternal spring would smile on the fortunate earth. The dream of the poet would be realised, and light would spread

"Through all the seasons of the golden year."

But, as observation shows, the contrary has taken place, since, besides its movements of diurnal rotation and annual revolution, the earth has its mobile axis, which is independently inclined and displaced. Thus, the material molecules of the planetary surface are not all at an equal distance from the centre; and, consequently, the earth is not a perfect sphere. It is, as D'Alembert has demonstrated, the bulging, equatorial portion which experiences, owing to the solar attraction, a retrograde movement, carrying onward the rest of the globe in a general march, called the precession of the equinoxes.