26 THE WORLD. 



very early observed that the sun and moon moved around the 

 earth with different velocities from the stars 1 , and that there were 

 certain bodies, five in number, which also appeared to be wan- 

 dering in the heavens, these were called planets, from a Latin 

 word meaning to wander, and were named in order, according to 

 their supposed distance from the earth, Mercury, Venus, Mars, 

 Jupiter, and Saturn. As soon as these wandering bodies were 

 closely observed, certain irregularities in their motion attracted 

 attention, instead of moving uniformly in a circle in the heavens, 

 like the sun, their paths were often broken, and even turned back, 

 as represented by the lines below, moving from a to b direct, i. c., 



in the order of the signs, from b to c, retrograde, or contrary to 

 their previous motion, at b and c, apparently still, or stationaiy for a 

 short time, and from c to d moving again direct. In addition to 

 these irregular movements, two of them were observed to always 

 remain in the neighborhood of the sun, viz. Mercury and Venus, 

 while Mars, Jupiter, and Saturn were often seen directly opposite, 

 rising when the sun was setting. Hence, in framing any theory, 

 it was necessary to account for these motions. 



All the early astronomers supposed that the earth was the centre 

 of the system, and that all the celestial bodies were revolving 

 around it. The only system of the world which attracted much 

 notice, was that of Ptolemy the great Egyptian king and philoso- 

 pher, called, from him, the Ptolemaic system. This is the 

 system which we would naturally adopt upon casual thought. 

 Here is the earth occupying the centre, and around it the moon is 

 supposed to be revolving not quite as fast as the sun, next comes 

 Mercury, then Venus, the Sun, Mars, Jupiter, and Saturn, beyond 

 the whole was supposed to be the grand prinwm mobile, a sphere- 



