ON THE APPEARANCES OF THE CELESTIAL BODIES. 527 



horizon less obliquely than any lesser circle parallel to it. (Plate XXXIV. 

 Fig. 490, 491.) 



The revolutions of the primary planets, combined with that of the earth, 

 necessarily produce the various relations, in which they are either in opposi- 

 tion or conjunction, with respect to each other or to the sun, and in which the 

 apparent motion is direct or retrograde, or the planet is stationary, accord- 

 ing to the directions and the comparative velocities of the real motions. If 

 the earth were at rest, the inferior planets would appear to be station;uy when 

 they are at the greatest elongation or angular distance from the sun; but, 

 on account of the effect of the earth's motion, Venus is stationary at an 

 elonga*^ion of about 29°, while her greatest elongation is between 45° and 

 48°. The greatest elongation of Mercury, in each revolution, is from 28-j° 

 to 17t°> according to the position of his orbit, which is very eccentric. All 

 these appearances are precisely the same as if the sun actually revolved round 

 the earth, and the planets accompanied him in his orbit, performing at the 

 same time their several revolutions round him; and the path which would 

 thus be described in the heavens, and which is of a cycloidal nature, re- 

 presents correctly the true positions of the planets with respect to tiie earth. 

 The apparent angular deviation from the ecliptic, or the latitude of the planet, 

 is also greater or less, accordingly as the earth is nearer or remoter to the planet, 

 as well as according to the inclination of its orbit and its distance from the 

 node. (Plate XXXIV. Fig. 492 . . 494.) 



The various appearances of the illuminated discs, especially of the inferior 

 planets, and the transits of these planets over the sun, depend on their 

 positions in their orbits, and on the places of the nodes, with respect to tlie 

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

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

 illuminated. Venus is brightest at an elongation of about 40° fron) the sun, 

 in that part of her orbit which is nearest to the earth; she then appears like 

 tlie moon when 5 days old, one fourth of her disc being illuminated; slie 

 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 15^° of Jier node at the time of conjunction, 



