LONGITUDE AND TIME 



3499 



LONGITUDE AND TIME 



JONGITUDE AND TIME. Distances on 

 le earth's surface are measured not only in 

 iles, but in degrees of longitude and in 

 me. As explained in the article immediately 

 eceding, the earth is divided into 360 north- 

 id-south sections, each one of which is a de- 

 ee of space, marked by imaginary lines called 

 eridians; also, a degree of longitude is the 

 stance between a point on any meridian and 

 le point directly , east or west on the next 

 meridian. Since the earth turns on its axis 

 ice in 24 hours, every point on its surface 

 ill describe a circumference (360) in that 

 >ace of time. In other words, it takes 24 

 3urs for the entire 360 of the earth's circum- 

 Tence to pass beneath the sun. Then, since 

 I hours of time are equivalent to 360 of space, 

 hour of time represents ^4 of 360 of space, 

 15 of space. See DEGREE. 

 This means that the sun apparently travels 

 irough 15 in one hour. How far, then, will 



travel in one minute and in one second? 

 learly there must be smaller divisions of 

 )ace measurement than the degree, just as 

 lere are smaller divisions of time than the 

 Dur. These divisions are minutes and sec- 

 ids of space, the symbols of which are small, 

 anting marks written to the right of the num- 

 sr; 15 minutes is 15', and 15 seconds, 15". 

 hese must not be confused with minutes and 

 ;conds of time, for they are as different as 

 Durs and degrees. The minute of space is % 

 ' a degree, and the second is %o of a minute. 

 7e have then the following table, the sign = 

 leaning is equivalent to: 

 24 hours of time = 3 60 of space 



1 hour of time = 15 of space 



4 minutes (i/ 15 hour) of time=rl of space 



1 minute of time = 15' of space 



1 second of time = 15" of space 



As a common point from which to reckon 

 ngitude the English-speaking world has se- 

 ated the meridian passing through Greenwich, 

 ingland, near London, where there has been 

 3r many years a great observatory. The longi- 

 ide of Greenwich is therefore 0. See GREEN- 



r ICH. 



Practical Applications. It follows, then, that 

 a distance expressed in degrees of lorfgitude 

 may also be expressed in measures of time, and 

 vice versa. In the solution of problems based 

 on the above principles, the following rules will 

 be applied : 



(a) To find the difference in longitude between 

 two places when both are east or both are west 

 of a given meridian, subtract the lesser longitude 

 from the greater ; if one is east and the otner 

 west, add the two longitudes. To obtain the cor- 

 rect difference when the sum exceeds 180, sub- 

 tract the sum from 360. 



(b) If the time of a place is given, to find the 

 time of a place east, add to the given time the 

 difference in time between the two places (see 

 (e) below). To find the time of a place west, 

 subtract from the given time the difference in 

 time between the two places. 



(c) To find the difference in time when the 

 difference in longitude is given, divide the differ- 

 ence in longitude, expressed in degrees, minutes 

 and seconds, by 15. The quotient will be the 

 difference in time expressed in hours, minutes and 

 seconds. 



(d) To find the difference in longitude when 

 the difference in time is known, multiply the dif- 

 ference in time, expressed in hours, minutes and 

 seconds, by 15. The result will be the difference 

 in longitude expressed in degrees, minutes and 

 seconds. 



(e) Since the sun seems to mtove fro'm east to 

 west, sunrise will occur earlier at all points east 

 and later at all points west of any given place. 

 Clock-time, therefore, will be later in all places 

 east and earlier in all places west of a given 

 meridian. See STANDARD TIME. 



ILLUSTRATIVE PROBLEM. When it is noon at 

 X, a town whose longitude is 71 3' 25" west, 

 what is the time at Y, longitude 2 20' 20" east? 



Since one of the two towns is west and the 

 other east of the principal meridian (at Green- 

 wich), the difference in longitude (that is, the 

 distance they are apart), is found by taking the 

 sum of their longitudes (Rule a). This differ- 

 ence is equivalent to 4 hours, 53 minutes, 35 sec- 

 onds of time ; the time difference is found by 

 dividing the longitude difference by 15 (Rule c), 

 and the divisor 15 is used because 1 hour of time 

 is equivalent to 15 of space. 



