moon’s passage from the quarter to the full would be accelerated, if the sun were to act on the earth and moon in a direction parallel to A S: and the obliquity of the sun’s action will still more increase this acceleration. For the action of the sun on the moon is oblique to the line S A the whole time of the moon’s passage from Q to T, and will carry the moon out of the line T V toward the earth. Here I suppose the time of the moon’s passage from Q to T so short, that it shall not pass beyond the line S A. The earth also will come a little short of the line I N, as was said before. From these causes the angle under X M W will be still farther lessened. 15. THE moon in passing from the opposition B to the next quarter will be retarded again by the same degrees, as it is accelerated before its appulse to the opposition. Because this action of the sun, which in the moon’s passage from the quarter to the opposition causes it to be extraordinarily accelerated, and diminishes the angle, which measures its distance from the opposition; will make the moon slacken its pace afterwards, and retard the augmentation of the same angle in its passage from the opposition to the following quarter; that is, will prevent that angle from increasing so fast, as otherwise it would. And thus the moon, by the sun’s action upon it, is twice accelerated and twice restored to its first velocity, every circuit it makes round the earth. This inequality of the moon’s motion about the earth is called by astronomers its variation. 16. THE next effect of the sun upon the moon is, that it gives the orbit of the moon in the quarters a greater degree of curvature, than it would receive from the action of the earth alone; and on the contrary in the conjunction and opposition the orbit is less inflected.