"Three years after this discovery, by which Mr. Bradley acquired very great reputation, he was appointed Lecturer in Astronomy and Physic, at the Museum at Oxford. He pursued his studies with equal application and delight; and in the course of his observations... he discovered that the inclination of the Earth's axis, upon the plane of the , was not always the same, but that it varied backwards and forwards some seconds, and that the period of these variations was nine years. This period seemed altogether unaccountable, as it could not be supposed to have any thing in common with the revolution of the Earth, which is performed in one year. Mr. Bradley, however, discovered the cause of this phenomenon in the Newtonian system of attraction. The first principle of that system is known to be, that all bodies mutually attract each other in the direct ratio of their masses, and in the inverse ratio of the squares of their distances. From this mutual attraction, combined with motion in a right line, Newton deduces the figure of the orbits of the planets, and particularly that of the Earth. If this orbit was a circle, and if the terrestrial globe was a perfect sphere, the attraction of the Sun would have no other effect than to keep it in its orbit, and would cause no irregularity in the position of its axis; but neither is the Earth's orbit a circle, nor its body a sphere; for the Earth is sensibly protuberant towards the equator, and its orbit is an ellipsis, which has the Sun in its focus. When the position of the Earth is such, that the plane of its equator passes thro' the centre of the Sun, the attractive power of the Sun acts only so as to draw the Earth towards it, still parallel to itself, and without changing the position of its axis, and this happens at the equinoxes. In proportion as the Earth recedes from those points, the Sun also goes out of the plane of the equator, and approaches that of one or other of the tropics; the semidiameter of the Earth, which is then exposed to the Sun, being no longer equal, the equator is more powerfully attracted than the rest of the globe, which causes some alteration in its position, and its inclination upon the plane of the ecliptic; and as that part of the orbit, which is comprized between the autumnal and vernal equinox, is less than that which is comprized between the vernal and the autumnal, it follows, that the irregularity caused by the Sun, during his passage through the northern signs, is not entirely compensated by that which he causes during his passage through the southern signs; and that the parallelism of the terrestrial axis, and its inclination with the ecliptic, will be a little changed. But though the irregularity is now accounted for, we are still at a loss for the cause of its happening in a period of nine years. This difficulty, however, will immediately disappear."