u8 POPULAR SCIENCE MONTHLY. 



not, in fact, generally received until the day of Newton, though it was 

 sufficiently established by the observations of Galileo and convincingly 

 by the calculations of Kepler. To actually demonstrate the rotation 

 of the earth on its axis we must have recourse to an elaborate experi- 

 ment like that of Foucault on the pendulum, or to comparisons of the 

 force of gravity in different latitudes; to demonstrate its revolution 

 round the sun it is necessary to measure the time required for light to 

 reach us from the distant planets, or to evaluate the aberration of the 

 light of the fixed stars. It was not easy for the sixteenth century to 

 make a decision. If the heliocentric theory were true, then the planet 

 Venus must show phases like the moon; but no phases could be seen. 

 It required Galileo's telescope to show them. Moreover, the fixed stars 

 must have annual apparent displacements in miniature orbits. None 

 such were visible; none were detected until 1837, when Bessel deter- 

 mined the parallax of a fixed star (61 Cygni) for the first time. Galileo 

 sought for them in vain ; so did Herschel ; so did other astronomers of 

 the eighteenth century with their splendid instruments. The concep- 

 tion of epicycles was retained in the ' De Eevolutionibus, ' and it seems 

 to us a blemish; to the contemporaries of Copernicus it was a mere 

 analytic device. Newton explains one of the inequalities of the moon 's 

 motion by an epicycle, in the 'Principia.' 



It is only when we thus consider in detail how the new ideas must 

 have presented themselves to the students of the sixteenth century that 

 we can comprehend the real obstacles in the way of their acceptance. A 

 genius like Kepler could receive them simply on their intellectual merits. 

 Men in general required time to change their point of view, and to 

 accept a novel and essentially disheartening theory. Ptolemy's system 

 of the world was compendious, comfortable, so to say, and easily under- 

 standed of the people. Man's central position in the universe flattered 

 his pride and allayed his fears. 



Peter the Lombard (1100-60) expresses the accepted view in its 

 baldest form : ' Just as Man is made for the sake of God, that is, that 

 he may serve him, so the Universe is made for the sake of Man, that is, 

 that it may serve him; therefore is Man placed at the middle point of 

 the Universe, that he may both serve and be served.' The new view 

 made man an outcast and placed him in immense and disquieting soli- 

 tudes. Pascal has phrased the new and anxious fear: 'Le silence 

 eternel de ces espaces infinis m'effraie.' 



Astronomers needed accurate tables of the planetary motions in 

 order to predict eclipses and conjunctions. The Alphonsine tables 

 were quite unsatisfactory. The theory of Copernicus was made the 

 basis of new tables — the Prutenic tables — by Eeinhold in 1551, and 

 they remained the standard until 1627, when the Kudolphine tables, 

 based on Kepler's theories and Tycho's observations, superseded them. 



