310 ADDRESS ON PRESENTING THE GOLD MEDAL OF THE [42 



These important objects have been steadily pursued at the Pulkowa 

 Observatory, under the guiding mind of its illustrious director, with an 

 energy and success which have placed that establishment in a position 

 with respect to sidereal astronomy, similar to that which our own obser- 

 vatory of Greenwich occupies with respect to the observation of the Moon. 



The order of date, as well as the nature of the subjects treated of, 

 leads me first to speak of M. Peters' paper on the constant of nutation. 

 But before proceeding to give an account of the paper itself, it may not 

 be out of place to advert rapidly to former researches respecting nutation. 



When Newton traced the precession of the equinoxes to its cause in 

 the attraction of the Sun and Moon on the protuberant equatoreal zone 

 of the terrestrial spheroid, he perceived that the Sun's action would likewise 

 cause a nutation of the Earth's axis, the period of which is half a year. 

 He contents himself with remarking that this nutation can be scarcely 

 sensible. 



In the same way, of course, the Moon's action produces a small 

 nutation, of which the period is half a month. Abstracting these nutations, 

 the tendency of the Sun's action is to make the pole of the equator move 

 in a circular arc about the pole of the ecliptic; and in a similar manner 

 the Moon's action tends to make the pole of the equator describe a circular 

 arc about the pole of the Moon's orbit for the time being. Now, as this 

 latter pole moves in a circle about the pole of the ecliptic in a period 

 of about nineteen years, it is easy to see that this will give rise to an 

 inequality in the rate of precession, and to a change of the obliquity of 

 the ecliptic, having the same period. 



It is curious, however, that Newton does not allude at all to this, 

 which constitutes by far the most important part of nutation ; and this 

 is the more remarkable, since the principles which he lays down in treating 

 of precession are quite sufficient to obtain, by means of very simple 

 geometrical reasoning, not only the law, but very approximately, the co- 

 efficients of the inequalities in the precession and obliquity due to this cause. 



The state of practical astronomy, however, in Newton's time, was not 

 sufficiently advanced to induce him to enter more fully into this subject; 

 and it was, consequently, reserved for the immortal discoverer of aberration 

 to detect these motions of the Earth's axis by means of his observations, 

 and then to trace them to their true cause. While discussing the obser- 

 vations which led him to the discovery of aberration, Bradley noticed that 

 the annual changes of declination of the stars did not exactly correspond 



