Page:Newton's Principia (1846).djvu/450
444 [Book III.
the mathematical principles
the sine of the mean inclination, to four times the radius; that is, seeing the mean inclination is about 5° 8½, as its sine 896 to 40000, the quadruple of the radius, or as 224 to 10000. But the whole variation corresponding to BD, the difference of the sines, is to this horary variation as the diameter BD to the arc Gg, that is, conjunctly as the diameter BD to the semi-circumference BGD, and as the time of 20797⁄10 hours, in which the node proceeds from the quadratures to the syzygies, to one hour, that is as 7 to 11, and 20797⁄10 to 1. Wherefore, compounding all these proportions, we shall have the whole variation BD to 33" 10'" 33iv. as 224 7 20797⁄10 to 110000, that is, as 29645 to 1000; and from thence that variation BD will come out 16' 23½".
And this is the greatest variation of the inclination, abstracting from the situation of the moon in its orbit; for if the nodes are in the syzygies, the inclination suffers no change from the various positions of the moon. But if the nodes are in the quadratures, the inclination is less when the moon is in the syzygies than when it is in the quadratures by a difference of 2' 43", as we shewed in Cor. 4 of the preceding Prop.; and the whole mean variation BD, diminished by 1' 21½", the half of this excess, becomes 15' 2", when the moon is in the quadratures; and increased by the same, becomes 17' 45" when the moon is in the syzygies. If, therefore, the moon be in the syzygies, the whole variation in the passage of the nodes from the quadratures to the syzygies will be 17' 45"; and, therefore, if the inclination be 5° 17' 20", when the nodes are in the syzygies, it will be 4° 59' 35" when the nodes are in the quadratures and the moon in the syzygies. The truth of all which is confirmed by observations.
Now if the inclination of the orbit should be required when the moon is in the syzygies, and the nodes any where between them and the quadratures, let AB be to AD as the sine of 4° 59' 35" to the sine of 5° 17' 20", and take the angle AEG equal to double the distance of the nodes from the quadratures; and AH will be the sine of the inclination desired. To this inclination of the orbit the inclination of the same is equal, when the moon is 90° distant from the nodes. In other situations of the moon, this menstrual inequality, to which the variation of the inclination is obnoxious in the calculus of the moon's latitude, is balanced, and in a manner took off, by the menstrual inequality of the motion of the nodes (as we said before), and therefore may be neglected in the computation of the said latitude.
By these computations of the lunar motions I was willing to shew that by the theory of gravity the motions of the moon could be calculated from their physical causes. By the same theory I moreover found that the annual equation of the mean motion of the moon arises from the various