528 LF.CTUHE XLIV. 



Otherwise she wijl pass either to the north or to the south of the sun, instead 

 of being immediately interposed between him and the earth. 



The phases and eclipses of the moon are very obviously owing to the 

 same causes; that part of the nroon only, on which the sun shines, being 

 strongly illuminated, although the remaining part is faintly visible, by- 

 means of the light reflected on it from the earth; it is, therefore, most easily 

 seen near the time of the new moon, when the greatest part of the earth's 

 surface turned towards the m )on is illuminated. The parts of the moon 

 which are immeaditely op|)osed to the earth, appear to undergo a libration, 

 or chang-e of situation, of two kinds, each amounting to about 7 degrees: the 

 one arising from the inequality of the moon's velocity in her orbit at different 

 times, the other from the inclination of the axis of her rotation to her orbit; 

 besides these changes, the diurnal rotation of the earth may produce, to a 

 spectator situated on some parts of it, a third kind of libration, or a change of 

 almost two degrees in the appearance of the moon at her rising and setting. 

 (Plate XXX IV. Fig. 495.) 



When the moon passes the conjunction, or becomes new, near to the node, 

 she eclipses the sun, and when she is full, or in opposition, in similar circum- 

 stances, she herself enters the earth's shadow. 1 he earth's shadow consists 

 of two parts, the true shadow, within which none of the sun's surface is 

 visible, and the penumbra, which is deprived of a part only of the sun's light; 

 the true shadow forms a cone terminating in a point at a little more than 34 

 limes the mean distance of tiie moon; the penumbra, on the contrary, consti- 

 tutes, together with the shadow, a portion of a cone diverging from the earth 

 without limit; but the only effect of this imperfect shadow^ is, that it causes 

 the beginning of a lunar eclipse to be incapable of very precise determina- 

 tion; for the limit of the darkened part of the moon, as it appears in the 

 progress of the eclipse, is that of the true shadow, very little enlarged by the 

 penumbra. The true shadow, where the moon crosses it, is about 80 minutes 

 in diameter, as seen from the earth, while the moon herself is only 30. This 

 shadow is not, however, wholly deprived of the sun's light; for the atmo- 

 spheric refraction inflects the light passing nearest to the earth, in an angle 

 of 66 minutes, and causes a great part of the shadow to be filled with light 

 of a ruddy hue, by means of which the moon remains still visible to us, the 



