32 THE WORLD. 



however to account for the irregularities observed in the planetary 

 motions. Copernicus might have easily perceived, and no doubt 

 did perceive, that the motion of the sun backwards in the heavens, 

 and to which we have alluded, was only apparent, and was due to 

 a real motion of our earth, which may be illustrated thus : 



Let S represent the sun, occupying the centre of the system, 

 and E the earth moving in an orbit around it. Now an observer 

 on the earth at E would perceive tbe sun S, apparently projected 

 against the heavens near the star B. If the earth was stationary, 

 then after 24 hours, turning around in the direction of the arrow, 

 i. e., frem Jeft to right, or west to east, (the north pole in the dia- 

 gram being supposed towards the eye) the sun would again appear 

 close to the star B, and the sun and stars would come to the 

 meridian or mid-heaven together. Now suppose the earth to have 

 moved forward in its orbit to A, and imagine the sphere of stars 

 figured in the diagram to be expanded to an infinite distance, it 

 will be easy to see that the sun and the star B, will no longer 

 come to the meridian together, the meridian being represented by 

 the black line on A, but that, on the supposition that the earth is 

 turning in the direction of the arrow, the sun would come to the 

 meridian, or this line, much later than the star, and would appear 

 among the stars at C. To explain tbe motions of Mercury, and 



