146 



NATURE 



[April 25, 191: 



TIME 



T SEA AND THE ASTRO- 

 NOMICAL DAY. 



IN spite of the stress of 'war, the British, French, 

 and Italian Admiralties found opportunity 

 last year to come to an important decision on the 

 question of timekeeping at sea. Hitherto the 

 general practice appears to have been to set the 

 ship's clocks to the local time corresponding with 

 the place where observations were made, and con- 

 tinue its use until further observations were 

 secured. In consequence, two vessels speaking 

 each other might record different times for their 

 meeting ; cases are not unknown where it has 

 been of legal importance to ascertain the exact 

 time of a death occurring at sea, which was a 

 matter of some difficulty oh the old system. It 

 has now been resolved to extend to the sea the 



Fig. I. — P. Vincent's design for chronometer dials. 



system which has been so widely adopted on land, 

 of keeping time which differs by an integral 

 number of hours from Greenwich time, the hour 

 being changed on crossing meridians , 15° apart. 

 In this connection it may be noted that there is 

 need of a short name for the regions that keep 

 the same time. The word "zone" is to be de- 

 precated, since both by root-meaning and by usage 

 it suggests a belt parallel to the equator. The 

 word "lune" has been adopted in works on 

 spherical trigonometry, but is apt to suggest a 

 connection with the moon. The French use the 

 somewhat cumbersome term "fuseau horaire." 

 Mr. T. C. Hudson suggests the term " douve "; 

 :t means a barrel-stave, which has some resem- 

 blance to the shape of the regions in question.' 

 Commandant P. Vincent has devised a new form 

 NO. 2530, VOL. lOl] 



of chronometer dial to facilitate the determination 

 of the ship's time; he describes and illustrates it 

 in Ea Nature for March 2 (see F'ig. i). There are 

 three hands, indicating day of month, Greenwich 

 hour, and minute respectively. Apart from the 

 month hand, which is of the nature of a luxury, 

 the new form of dial could be readily adopted for 

 all chronometers. It has five concentric circles of 

 graduation : first, the degrees of longitude, 

 counted from 0° to 180*^ in each direction ; second, 

 the hours to be added or subtracted ; third, the 

 graduations of the minutes of time; fourth, the 

 Greenwich hours, reckoned from oh. (midnight) to 

 23h. ; fifth, the days of the month; inside these 

 the compass points are indicated, but they can 

 be omitted if desired. 



The discussion on time at sea has incidentally 

 reopened a larger question, w^hich was mooted 

 some thirty years ago, but shelved for a 

 time. This is whether the use of the astro- 

 nomical day, commencing at noon, might 

 not be discontinued, and 'the ciyil day, 

 commencing at midnight, extended to 

 astronomy. Tliis matter has been under 

 informal discussion for several months, ' 

 and in the opinion of the present writer 

 the general feeling is in favour of the 

 change, though there are some names of 

 great weight on the other side. The 

 astronomical day goes back at least to 

 the time of Ptolemy ; it is based on the 

 obvious principle that the bulk of observa- 

 tional work is done g^t night, so that the 

 night should be kept as an unbroken 

 unit. But this point is not gained with- 

 out appreciable inconveniences ; most 

 astronomers must have felt a considerable 

 amount of mental worry in having a dif- 

 ferent calendar date for all occurrences 

 between midnight and noon, according as 

 we are considering their astronomical or 

 their civil aspect; even the month or year 

 is sometimes affected. There is, more- 

 over, some confusion as to whether the 

 astronomical day begins with mean or 

 apparent noon ; the Nautical Almanac 

 uses both systems in different sections, so 

 that several minutes each day form a sort 

 of No-man's-land, being claimed for different 

 dates on different pages of the almanac ; there 

 would be no similar confusion at midnight, ap- 

 parent midnight being a meaningless phrase. 



Both the British and French Admiralties are 

 agreed that the use of the civil day would be a 

 convenience to navigators. The French have 

 already decided to adopt the civil day in their 

 abridged Seamen's Almanac from the year 1920. 

 A few British astronomers have suggested that 

 our abridged almanac should be changed, but not 

 the larger one. This would lead to great risk of 

 error, since the larger almanac is often used at 

 sea ; the sailor has a claim to consideration before 

 the professional astronomer, since the latter has 

 more leisure to make his calculations, and errors, 

 if made, are less disastrous. 



