Page:The American Cyclopædia (1879) Volume XI.djvu/817

 MOON 799 The former period is sometimes called the side- real month, the latter the synodic month. When not eclipsed, she always presents to the sun an illuminated hemisphere ; her phases depend on the amount of that hemisphere turned toward the earth. If the earth is directly between her and the sun, we see all of it ; if she is be- tween us and the sun, we see none of it ; if she is midway between these positions, we see half of it. In the first position, she is said to be in opposition ; in the second, in conjunc- tion ; in the third, in quadrature, or quarter ; and her phases, in order, are known familiar- ly as new, crescent, half-moon, gibbous, and full. From the constancy of the physical fea- tures of the moon's disk, it is evident that she always presents to us the same hemisphere. To do this she must turn upon her axis pre- ly once while making one revolution in her )it. This appears to be the general law of motions of the satellites. But it is not lite accurate to say that the moon constantly resents the same hemisphere to every ob- rer upon the earth. Her axis of rotation ing inclined one degree and a half to her rbit, and maintaining the same general direc- in space as she moves round the earth, she >pears to nod backward and forward in an of about 13 in the course of every revo- ition, exposing to view the regions just be- yond her N. and S. poles alternately. Nor is Full Moon, from Photographs taken by Prof. H. Draper, New York. ris all. As the moon's orbit, like that of Jvery other planetary body, is an ellipse, her rbital velocity is not uniform, being most rapid when she is nearest the earth. Thus she )metimes gets ahead of her mean place, and sometimes lags behind it ; and as her axial ro- tation is absolutely uniform, we are enabled to look over her edge, so to speak, now on the eastern and now on the western side. The arc through which she oscillates in this way amounts to more than 15. And again, the 572 VOL. XL 51 constancy of the direction of her hither hemi- sphere is to be referred to the earth's centre, so that the observer, situated upon the extrem- ity of the earth's radius, views her from an elevation of nearly 4,000 m. ; and when she is in the horizon it is plain he can look over her elevated edge, as it were. The oscillation thus occasioned is much smaller than either of the others, amounting only to about 2. These several exposures are called the moon's libra- tions : the first her libration in latitude ; the second her libration in longitude; the third her diurnal libration. The absolute maximum librations from the moon's mean position are as follows : libration in latitude, 6 44' ; in lon- gitude, 7 45'; diurnal, 1 H'. If the whole surface of the moon be regarded as equal to 10,000, then instead of seeing only 5,000 parts, as we should do if there were no libration, our range of view extends over 5,802 parts with- out taking the diurnal libration into account, and over 5,889 parts if diurnal libration be considered. So that only 4,111 parts of the moon out of 10,000 remain absolutely con- cealed from human ken. To the casual ob- server the motions of the moon in different sea- sons of the year seem exceedingly irregular. She is sometimes seen, at the full, coursing along a circle which passes near the zenith in these latitudes, and sometimes, in the same phase, along an arc low down in the southern sky. It is plain that this is mainly owing to the inclination of the earth's equator to the ecliptic; but there is a large residual effect which is due to the inclination of the moon's orbit to the plane of the ecliptic, amounting to 5 8', so that during one half of her orbit she is south of the sun's annual path, and during the remaining half north of it. The points where she crosses the ecliptic are known as her nodes ; that at which she passes from the southern to the northern side of the line is called her ascending node, the other her de- scending node. If the ecliptic were a line of light ever conspicuous in the sky, and the moon's path intersecting it also a conspicuous line of light, the place of crossing would be seen to be different every month, being re- moved further and further to the westward at intervals of about three diameters of the moon. This at least is the average rate of the motion ; for the motion is not only not uni- form, but is at times reversed. It is known as the retrograde motion of the nodes; the period of completing the whole circuit of the ecliptic is 18-5997 years. The orbit of the moon being an ellipse, having the earth at one of its foci, her distance varies in different parts of her monthly course. The nearest point of her orbit is called perigee, the furthest apogee ; the two are known as apsides. These points are not fixed, but move forward (on the whole) from west to east, occupying succes- sively every position in the circumference of the ellipse in the course of 8-8505 years. These two remarkable motions, viz., of the nodes and