82 ASTRONOMY. 
because he is observed to the right of the stars at z, seen in a direction, by, 
parallel to az. Therefore Mars, at the time of opposition, retrogrades. 
After the earth has arrived at b, Mars is at B, and consequently no longer 
exactly opposite to the sun, S; but as the stars seen in the direction by 
culminate about midnight, he will then have passed the meridian and will 
be near his setting, and consequently will be seen in the evening sky. 
Two months after opposition, the earth being at c, and Mars at C, the 
retrograde motion of the latter will have ceased, since the straight lines 
connecting b, B and c, C, are nearly parallel; consequently to an observer at 
the earth, the planet will appear stationary for some days. At a later 
period the motion of Mars will be direct. Two months later, Mars, still 
moving direct, will be at D, and the earth at d, so that now the lines c C, 
d D, form the angle Cd’D. It is evident that the directions dS, dD, from 
the earth to the sun and to the planet, form nearly a right angle to each 
other, and consequently that about this time Mars at sunset must be near 
the south, and must set about midnight. 
At the end of the tenth month, when the earth is at g, Mars has not 
completed half his apparent course in the heavens, as shown by his not 
having reached a point opposite to the stars seen in the direction gz. The 
distance of Mars from the sun, however, is apparently only the angle GgS; 
as at this time an observer at g, looking towards the sun and planet, sees 
the latter to the left or eastward of the former. Mars must appear in 
the evening sky, after sunset, and evidently much smaller than at time of 
opposition. 
At the expiration of a full year, the earth being at h, and the planet at 
H, the straight line, AH, shows that Mars has completed rather more than 
half the apparent circuit of the heavens, since the stars in the direction z 
stand nearly opposite to him. Consequently the conjunction of Mars with 
the sun (¢<¢©) has not yet taken place, as, from an inspection of fig. 25, it 
will be seen that the sun still appears to the right of Mars or H. 
Two months later Mars is seen early in the morning sky, for since from 
i, the position of the observer, the planet is seen to the right of the sun, at I, 
he must evidently rise before the sun. By continuing this consideration, with 
the assistance of fig, 25, we shall soon find that Mars, shortly before his new 
opposition, again beeomes retrograde, and that the phenomena before 
observed must all succeed each other again in the same order. 
From all that ‘has preceded it follows, without further explanation, that 
the inferior planets have an inferior and a superior conjunction, but no 
opposition ; that the superior planets have a conjunction and an opposition, 
never two conjunctions; and, finally, that while the inferior planets are 
never visible in the heavens at midnight, the superior may be seen at any 
hour of the night. 
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