LIGHT. 



shall then, as Boemer did, observe the moments at which the 

 eclipses occur and compare them with the moments registered 

 in the table. 



Let the earth be supposed at A, at the commencement of these 

 observations, where it is nearest to Jupiter. When the earth 

 has moved to B, which it will do in about six weeks, it will be 

 found that the occurrence of the eclipse is a little later than the 

 time registered in the table. When the earth arrives at c, which 

 it will do at the end of three months, it will occur still later than 

 the registered time. In fact, at c the eclipses will occur about 

 eight minutes later than the registered time. At D they will be 

 twelve minutes later, and at E sixteen minutes later. 



By observations such as these, Eoemer was struck with the 

 fact that his predictions of the eclipses proved in every case to 

 be wrong. It would at first occur to him that this discrepancy 

 might arise from some errors of his observations ; but if such 

 were the case, it might be expected that the result would betray 

 that kind of irregularity which is always the character of such 

 errors. Thus it would be expected that the predicted time 

 would sometimes be later, and sometimes earlier than the 

 observed time, and that it would be later and earlier to an 

 irregular extent. On the contrary, it was observed during an 

 interval of little more than six months which the earth took to 

 move from A to E, that the observed time was continually later 

 than the predicted time, and moreover, that the interval by which 

 it was later continually and regularly increased. This was an 

 effect too regular and consistent to be supposed to arise from the 

 casual errors of observation ; it must have its origin in some 

 physical cause of a regular kind. 



The attention of Eoemer being thus attracted to the question, 

 he determined to pursue the investigation by continuing to 

 observe the eclipses for another half year. Time accordingly 

 rolled on, and the earth transporting the astronomer with it, 

 moved from E to r. On arriving at F, and comparing the 

 observed with the predicted eclipse, it was found that the 

 observed time was now only twelve minutes later than the pre- 

 dicted time. Soon after the expiration of the ninth month when 

 the earth arrived at G, the observed time was found to be only 

 eight minutes later ; at H it was only four minutes later, and 

 finally, when the earth returned to its first relative position 

 with the planet, the observed time corresponded precisely with 

 the predicted time.* 



From this course of observation and inquiry it became 



* The exact interval is 398 days, the synodic period of Jupiter. 

 198 



