ERA OF COPERNICUS, TYCHO BRAHE, KEPPLER, AND GALILEO. 21 



could now be offered in proof of the Copernican doctrine, there is an air of truth about it 

 sufficient to command the assent of a thoughtful mind. Proceeding upon the assumption, 

 which was universally admitted, that the earth is a mere point when compared with the 

 distance of the fixed stars, he very naturally remarked upon the improbability of such a 

 vast circumference revolving in twenty-four hours, instead of the infinitesimal point by 

 which the whole phenomena would be equally as well explained. It had been urged in 

 support of the earth's immobility, that, if it revolved on its axis, objects on its surface 

 would be scattered and dispersed in space by the extreme rapidity of the motion over- 

 coming the force of gravity. He did not see the true reply to this, that such effects 

 would not take place unless the velocity of rotation was greater than the force of gravity, 

 which was an arbitrary assumption, but still he reasonably turned the argument against 

 the objector by observing that the diurnal revolution of the sphere of the universe would 

 be far more likely to derange the situations of the heavenly bodies, and produce their 

 displacement. " Why, then," he exclaims, " do we hesitate to give to the earth the 

 mobility suitable to its form, rather than that the universe, whose bounds we do not and 

 cannot know, should revolve ? Why should we not confess that the diurnal revolution is 

 apparent only in the heavens, and real in the earth ? Thus JEneas, in Virgil, exclaims, 



' Provehimur portu, terraeque, urbesque recedunt.' 



Since, while the ship glides tranquilly along, all external objects appear to the sailors to 

 move in proportion as their vessel moves, and they alone, and what is with them, seem to 

 be at rest." 



The other conditions of the problem of the celestial motions, the sun's path in the 

 ecliptic, are as exactly answered by the supposition of the earth's orbital motion. The 

 annual revolution of the earth round the sun causes the apparent annual revolution of 

 the sun round the earth ; and when we consider the vast magnitude of the solar orb, and 

 the enormous waste of force implied in moving the greater body around the less, when 

 precisely the same effects are produceable by moving the less about the greater, our 

 common sense is at once enlisted in favour of the latter hypothesis as Nature's " wise and 

 frugal" plan. The apparent eccentricities of the planets likewise, their direct and retro- 

 grade movements, that mysterious puzzle which called the epicycles of the Ptolemaists 

 into existence, are explained upon their own principle of two combined motions : an 

 observer on the earth in ceaseless translation sees them performing a similar orbital 

 course, and apparent irregularity and involution are the consequences of the combined 

 prosecution of direct and regular paths. Suppose s the sun, A B c D part of the earth's 



orbit in the direction of the arrow, a deb part of the orbit 

 of a superior planet, and M N an arc of the celestial sphere. 

 '* When the earth is at A, and the planet at , a terrestrial 

 spectator will see it projected to a place in the heavens at 

 E. The angular motion of a superior planet being less 

 rapid than that of an inferior, when the earth is at B the 

 planet may be supposed to be at d, and its place will be 

 projected in the heavens at F, thus apparently retrograd- 

 ing in the sphere from E to F, while accomplishing 

 the direct movement from a to d. The next movement 

 of the earth, B c, and of the planet, d c, will produce a 

 further retrogradation of the latter in the heavens from F to G ; but, when the earth has 

 arrived at D, and the planet at &, the retrocession of the planet will appear to have ceased, 

 and the direct movement, G H, to have taken place. As both the earth and the planet 

 proceed in their orbits, the planet will appear stationary among the fixed stars, then to 



c 3 



