Page:Popular Astronomy - Airy - 1881.djvu/216

202 the attractions is about $1⁄30$ of the whole attraction of the moon. But for the sun, the proportion of distances is nearly as 24,000 to 23,999, and then the difference of the attractions is about $1⁄12,000$ of the whole attraction of the sun. The consequence is, that that difference of attractions of the moon is actually three times as great as that of the sun, and the precession produced by the moon is three times as great as that produced by the sun.

This is the mechanical explanation of the precession of the equinoxes. It was discovered as a fact by Hipparchus, a Greek Astronomer, one hundred and fifty years before the Christian era; it has been recognized ever since by all astronomers, and is now known with very great accuracy; and, in all probability when Sir Isaac Newton first applied the theory of gravitation to the explanation of the movements of the solar system, the explanation of this discovery of Hipparchus' was one that struck his mind, and that of his contemporaries, more than any other.

The next subject which I pointed out as an important one in connection with observations, was nutation, and I described it in this way: that nutation is a want of uniformity in precession. For the explanation of precession, I had taken the position of the earth at one of the solstices. At this time the earth's equator is much inclined to the line connecting the sun with the earth's centre, and the precession of the equinoxes is going on very rapidly. But if the earth were in the equinoxial position, then the sun would shine equally on the North and South Poles, and the protuberance of the earth would be directed exactly to the sun; and the action of the sun upon that protuberance would not tend to change the position of the globe. From these causes it will be seen