80 ASTRONOMY. 
stars never set ; and Oo, the circle of perpetual occultation, between which 
and the invisible pole p the stars never rise. All stars situated between 
these two circles will be sometimes visible and sometimes invisible. Thus, 
the star S will be seen when in that part, ABa, of its diurnal circle above 
the horizon, and will be invisible when in the portion ADa. Furthermore, 
the same star will, in the diurnal rotation of the heavens, come back to the 
same meridian every twenty-four hours, and consequently as the daily rota- 
tion of the heavens is uniform, the interval of sidereal time between the 
arrival of the star at the meridian of two different places, may be ex- 
pressed by the difference of longitude of the two places. On the other 
hand, the interval expressed in the sidereal time between the arrival of two 
different stars at the same meridian, is measured by their difference of right 
ascension, so that here we find one reason for dividing the equator into both 
degrees and hours. We find, also, from an inspection of figs. 13 and 2, pl. 
6, that the altitude of the pole at any place is its geographical latitude, and 
that the sum of the altitudes of the pole and the equator is always for the 
same place equal to 90°. Likewise we find that every star attains the 
greatest height above the horizon at its culmination, and that all stars lying ~ 
within the circle of perpetual apparition, cross the meridian twice above 
the horizon, once above the pole, and once below it; the one being the 
upper culmination or transit, the other the lower. 
The Apparent Course of the Superior and Inferior Planets. 
In the Copernican system, the inferior planets, Mercury and Venus, which 
are nearer the sun than the earth, are distinguished from the superior, Mars, 
Vesta, Astrea, Juno, Ceres, Pallas, Jupiter, Saturn, Uranus, and Neptune, 
which are more remote. ‘This distinction is very proper, as the phenomena 
attending the courses of the inferior and superior planets, are in many 
respects essentially different from each other. The reason is, that we view 
the planets from our earth, which itself revolves around the sun at a differ- 
ent rate from the rest, and therefore we see them at very different dis- 
tances. We observe therefore not the true, but the apparent courses of the 
planets, which will now be explained. It must ever be kept in mind, how- 
ever, that the fixed stars being at almost an infinite distance from our earth, 
their rays must always reach it in parallel lines. In the first place, to illus- 
trate the apparent course of an inferior planet, let ACEG, in fig. 24, repre- 
sent the orbit of Venus, aceg? that of the earth, and S the sun. Since the 
distance of Venus from the sun is about three-fourths that of the earth, and 
since she traverses her orbit in 74 months, then, if in fig. 24 we divide her 
orbit into 5, and that of the earth into 8 equal parts, one of these will repre- 
sent the space traversed by each in 14 months. If, when Venus is at A, the 
earth is at a, the former is said to be in inferior conjunction with the sun 
(2 40), that is, on the same side of the sun with the earth. Her apparent 
diameter is here the greatest, although actually invisible (fig. 21); at the 
expiration of three-fourths of a month, Venus is at B, and the earth at 0. 
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