APPEARANCES OF THE CELESTIAL BODIES. 41!) 



their positions in their orhits, and on the places of the nodes, with respect 

 to the earth. Jupiter, Saturn, and the Georgian planet, are so remote in 

 comparison of the earth's distance from the sun, that they appear always 

 fully illuminated. Venus is brightest at an elongation of ahout 40 from 

 the sun,* in that part of her orhit which is nearest to the earth ; she then 

 appears like the moon when 5 days old, one fourth of her disc being illu- 

 minated ; she casts a shadow, and may even be seen in the day time in our 

 climates, if she happens to be far enough north ; a circumstance which 

 occurs once in about 8 years. In order that there may be a transit of 

 Venus over the sun, she must be within the distance of l.J of her node at 

 the time of conjunction, otherwise she will 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 ;t that part of the moon 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 moon is illuminated. The parts of the 

 moon which are immediately opposed to the earth, appear to undergo a 

 libration, or change 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 XXXIV. 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 

 circumstances, she herself enters the earth's shadow. The 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 3 times the mean distance of the moon ; the penumbra, 

 on the contrary, constitutes, together with the shadow, a portion of a cone 

 diverging from the earth without limit ; but the only effect of this imper- 

 fect shadow is, that it causes the beginning of a lunar eclipse to be incapa- 

 ble of very precise determination ; 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 atmospheric refraction inflects 

 the light parsing 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 



* Halley, Ph. Tr. 1716, p. 466. Kies, Hist, et Mem. de Berlin, 1750, 218. 

 t Kastner on the Phases of the Moon, Com. Gott. 1780, Hi. M. 1. 



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