ORBIT. The moon revolves round the earth in an elliptic orbit with the earth in one focus; the eccentricity of the ellipse hying 0.05491. or inane than times that of the earth's orbit. The plane of her orbit does not coincide with the ecliptic, but is inclined to it at an angle of 5° 8' 40", and intersects it in two opposite points, which are called the nodes (q.v.). Were the moon's orbit a true ellipse, which. owing to various irregularities known as perturbations (q.v.), it is not, the lunar theory (q.v.) would be exceedingly simple; but these perturbations cause in the ease of the moon a distinct and well marked deviation from her previous course in a single month. The retrogradation of he• nodes along the ecliptic eauses a continual change in the plane of her orbit. so that if, during one revolution round the earth, she occults certain stars, at the next revolution she may pass to one side of them, and Will remove farther and farther from them in each successive revolution. Owing to this continual change of her orbit, the moon in course of time passes over or occults every star situated within 5' 24' of the ecliptic. The
motion of the nodes is so rapid that they perform a complete circuit of the orbit in 18.6 years. ...\nother important change in the moon's orbit is the revolution of the line of apsides (q.v.), by which the perigee and apogee are continually changing their position relative to the earth and sun. This revolution is more than twice as rapid as that of the nodes. being performed in 8.85 solar years. This motion is analogous to the perturbations of planetary orbits. and its nature and origin are treated in the article PERTURBA TIONS. Its effect upon the moon is to produee variation in her distance from the earth, inde pendent of that produced by her elliptic motion. Among the further disturbances or perturbations of the lunar orbit we may mention the evection (q.v.), which may displace the nnoon's•ilocition in sky 1° 16' 27", the variation, which may amount to 39' 31". and the annual equation, whose maximum is 11' 9". See LUNAR THEORY.