THE MINOR PLANETS. 145 



will he seen under a corresponding variety of phases, in short, it will undergo 

 all the changes which the moon presents in its monthly course round the earth, 

 as represented in the figure. 



When near the point behind the sun, it will be nearly full, or gibbous ; and 

 when near the point where its dark hemisphere is turned to the earth, it will 

 be a crescent. In a certain intermediate position it will be halved, and will pass 

 through all the other phases. 



In making its circuit round the sun, it will be seen alternately at the east and 

 at the west of that luminary, separating from it in each direction to an extent 

 limited by the magnitude of its orbit round the sun. When it is at the west of 

 the sun, it sets before the sun, and rises before the sun. It cannot, in that case, 

 be seen in the evening ; but if it be separated from the sun by a sufficient dis- 

 tance, it will rise so early as to anticipate the light of the morning which 

 precedes the sun's rays, and may then be seen as a morning star. On the other 

 hand, when it is at the east of the sun, it rises after the sun. and sets after it. 

 It cannot, therefore, be seen in the morning ; but provided it be sufficiently dis- 

 tant from the sun to remain above the horizon until the darkness is sufficient to 

 render it visible, it will be seen as an evening star. 



The orbit of Mercury is so limited in its breadth, compared with the distance 

 of the earth from the sun, that even when that planet is at its greatest apparent 

 distance from the sun, it sets in the evening long before the end of twilight ; and 

 when it rises before the sun, the latter luminary rises so soon after it that it is never 

 free from the presence of so much solar light as to render it extremely difficult 

 to see the planet with the naked eye. In short, Mercury is seldom seen at all, 

 except with a telescope. It is said that Copernicus himself never saw this 

 planet. 



OF THE PLANET VENUS. 



The planet VEXUS is, on many accounts, more favorably circumstanced for 

 telescopic observation than Mercury. Its diameter is nearly equal to that of 

 the earth, and nearly three times as great as that of Mercury. Its distance 

 from the sun being about seventy millions of miles, it separates itself in its pe- 

 riodical course so widely from the sun, that when it is east of the sun it re- 

 mains above the horizon in the evening after night-fall ; and when it is west of 

 the sun' it rises in the morning so long before the hour of sunrise that it is dis- 

 tinctly visible. Owing to the absence of the solar light, it forms, therefore, 

 the object with whiofc every one is familiar, under the names of the morning and 

 evening star. It is subject, by the operation of the same causes, to the sajue 

 variety of appearances as Mercury. When it is nearly between the earth and 

 the sun it appears a thin crescent, and when beyond the sun it appears full ; ar.l 

 in the intermediate positions exhibits, like Mercury, all the variety of phases 

 of the moon. 



DIURNAL MOTION OF VENUS AND MERCURY. 



One of the most interesting objects of telescopic inquiry regarding the con- 

 dition of the planets is, the question as to their diurnal rotation. In general, 

 the manner in which we should seek to ascertain this fact would be, by exam- 

 ining with powerful telescopes the marks observable upon the disk of the planet. 

 If the planet revolves upon an axis, these marks, being carried round with it 

 would appear to move across the disk from one side to the other ; they would 

 disappear on one side, and, remaining for a certain time invisible, would reap- 

 pear on the other, passing, as before, across the visible disk. Let any one 



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