THE CONQUEST OF THE ZONES 



on reflection it may seem less and less obvious as to just 

 what test was available through which the exact dif- 

 ference in time between Greenwich, at which the watch 

 was originally tested, and local time at the station in the 

 West Indies could be determined. There are, however, 

 several astronomical observations through which this 

 could be accomplished, and in point of fact the com- 

 parative times and hence the precise longitudes at many 

 points on the Western Hemisphere — and indeed of all 

 portions of the civilized globe — were accurately known 

 before the day of the chronometer. 



One of the simplest and most direct means of testing 

 the time of a place, as compared with Greenwich time, 

 is furnished by observation of the occultation of one of 

 the moons of Jupiter. By occultation is meant, as is 

 well known, the eclipse of the body through passing into 

 the shadow of its parent planet. This phenomenon, 

 causing the sudden blotting out of the satellite as viewed 

 from the earth, occurs at definite and calculable periods 

 and is obviously quite independent of any terrestrial 

 influence. It occurs at a given instant of time and would 

 be observed at that instant by any mundane witness to 

 whom Jupiter was at that time visible. If then an ob- 

 server noted the exact local time at which occultation 

 occurred, and compared this observed time with the 

 Greenwich time at which such occultation was pre- 

 dicted to occur, as recorded in astronomical tables, a 

 simple subtraction or addition will tell him the difference 

 in time between his station and the meridian at Green- 

 wich; and this difference of time can be translated into 

 degrees of longitude by merely reckoning fifteen degrees 



[29] 



