THE PROOF OF THE EARTH'S MOTION 285 



in opposition to Jupiter it is about a hundred and eighty million 

 miles farther away than when it is in conjunction. If the trans- 

 mission of light is not instantaneous, then the discrepancy in 

 the observed times of the occultation will represent the time 

 it takes light to cross the diameter of the earth's orbit. This 

 Roemer estimated at twenty-two minutes. The speed of light, 

 then, will be one hundred and eighty million miles divided by 

 twenty-two minutes that is, about eight million miles per 

 minute. This is Roemer's explanation ; he presents it in a 

 paper to the French Academy in November 1675. 



The history of science is full of fatuities. Roemer's dis- 

 covery was laughed at by some, mostly ignored by others. 

 Perhaps there was some reason. A few years later in the 

 Principia, the disturbing influence of the planets and satellites 

 one upon the other was disclosed. Newton records that the 

 chapter on the inequalities of lunar motion cost him more 

 labour than all the rest of the book. Jupiter had then four 

 known satellites ; they mutually pull each other about. Their 

 motions represent one of the most difficult problems in astro- 

 nomy. This might explain the inequality in the duration of 

 their eclipses. 



Then, too, a speed of a hundred thousand miles or more 

 per second was something too vast for the mind easily to grasp. 

 Such a speed did not, however, appear absurd to Newton, for 

 at least in the second edition of his Optiks he utilised 

 Roemer's method, and calculated the time for the light of the 

 sun to reach the earth at seven and a half minutes. This was 

 an error only of a little more than half a minute ; but the close 

 approximation was an accident ; he set the sun at only seventy 

 million miles away. Yet even so great an authority could not 

 induce general acceptance of Roemer's theory. It was another 

 observer, travelling a widely different path, who was to confirm 

 his results in a singular way. 



As far back as 1667 Picard, the French astronomer whose 

 measures of the earth were to confirm Newton's discovery of 

 the law of attraction, had observed a periodical movement of 

 the pole-star to the extent of about 20*. This, he saw, could 

 neither be the effect of parallax nor of refraction ; it was 

 very regular, and extreme at opposite seasons of the year. He 

 could not divine the cause, neither could his young protege 



