ECLIPTIC, obliquity of, is the angle which its plane makes with that of the equinoctial. The inclination of the e quator to the ecliptic is measured by the arch of a great circle intercepted between their poles, which was taken with very great accuracy by Dr. Maskelyne, in the year 1769, and found to be 23° 28' 10", or 46944. It was formerly found by Dr. Bradley to be 23° 28' 30", who sup posed that there was a gradual approach of the ecliptic to the equinoctial, at,the rate of 1' in 100 years. The mean obli quity of the ecliptic is augmented by 9", when the moon's ascending node is in the vernal equinox. It is, on the contra ry, diminished 9",. when the node is in the autumnal equinox, and it is equal to the mean, when the node is in the co lure of the solstices. This change of the inclination of the earth's axis to the plane of the ecliptic was called the nutation of the axis by Sir Isaac New ton.
Dr. Bradley discovered a general and periodical motion in all the stars, which alter a little their relative situations. To
form an idea of this motion, let us sup pose that each star describes annually a small circumference parallel to the eclip tic, whose centre is the mean position of the star, and whose diameter, as seen from the earth, subtends an angle of about 40" ; and that it WAS in that circum ference as the sun in its orbit, but so that the sun always precedes it by 90°. This circumference, projected upon the sur face of the celestial sphere,- appears un der the form of an ellipse, more or less flattened, according to the height of the star above the equator, the smaller axis of the ellipse being to the greater axis as the sine of that height to the radius. These periodical movements of the stars have received- the name of aber rations of the fixed stars. See ABER.