niral motion, an unanswerable demonstration is obtained of the reality of that 

 motion. 



We have seen that the observation of the sun establishes demonstratively 



his alternative either that the earth revolves round the sun annually, or that 



he sun revolves round the earth annually. There is no other motion which 

 would be consistent with the phenomena. Now, the effect on the stars called 



he aberration of light, just explained, proves that of the sides of this alterna- 



ive, that which must be adopted is the motion of the earth. 



There is an instinct of the human mind which leads it to anticipate discov- 

 eries. The grounds on which the annual motion of the earth and the stationa- 

 ry position of the sun are demonstrated, are modern. The theory of gravita- 



ion dates only from the era illustrated by Newton. The discovery of the ab- 

 erration of light is still more recent ; and yet the first suggestion of the annual 

 motion of the earth, and the stationary position of the sun, dates as far back as 



he era of Pythagoras. It is true that this hypothesis did not obtain general 

 assent until it was urged by the sagacity of Copernicus, and reinforced by the 

 eloquence and talents of Galileo and Kepler. But still it affords an example 

 of one of those wonderful anticipations of human intellect which leads us irre- 

 sistibly back to the impression that the mind is itself an emanation of the Divine 

 spirit which was breathed into our nostrils when He who created us gave us 



he breath of life, and made us a living soul. 



THE EARTH'S DIURNAL MOTION. 



W T hile the earth revolves annually round the sun, it has a motion of rotation 

 at the same time upon a certain diameter as an axis which is inclined from the 

 perpendicular to its orbit at an angle of 23, 28'. During the annual motion 

 of the earth this diameter keeps continually parallel to the same direction, and 

 the earth completes its revolution upon it in twenty-three hours and fifty-six 

 minutes. In consequence of the combination of this motion of rotation of the 

 earth upon its axis with its annual motion round the sun, we are supplied with 

 the alternations of day and night, and the succession of seasons. 



When the globe of the earth is in such a position that its north pole leans 

 toward the sun, the greater portion of its northern hemisphere is enlightened, 

 and the greater portion of the southern hemisphere is dark. This position is 

 represented in the annexed figure, 6, where C is the north pole, and D the 



fc 



& 



south pole. As the earth revolves upon its axis, the parallels of the equator 

 are unequally divided by the circles of light and darkness : the greater segment 

 of each of them being illuminated, and the lesser segment dark. The days 



