174 ^^^ POPULAR SCIENCE MONTHLY. 



OX THE DISTEIBUTION OF STAND AED TIME IE THE 



UNITED STATES. 



By EDWAED S. HOLDEN, 

 united states naval obsekvatory, washington. 



FOR the ordinary purposes of life in a state of society which is 

 not yet complex, a very simple system of recording the lapse 

 of time is sufficient. Sunrise and sunset are local phenomena, which 

 from the earliest times forced themselves upon the attention of every 

 one, and which throughout the early centuries sufficed for the divi- 

 sion of time. A further division of the duration of the day (as 

 defined by the continuance of sunlight) was obtained by noting the 

 time of noon, and there is no historic period known in which the 

 method of obtaining a rough apjjroximation to this instant by means 

 of the shadow of a vertical rod or pillar was not understood. Prob- 

 ably the observation of such a gnomon or style constituted the first 

 step in astronomy of precision^ as distinguished from that astronomy 

 in which numbers do not play the most important part. The instant 

 so determined is technically called the instant of apparent noon at 

 any place, and it marks the moment when the sun is highest above the 

 horizon and on the meridian.' 



Until within a hundred years this apparent time, that is the time 

 marked by the angular distance of the sun from the meridian of any 

 place, was the system universally adopted. A watch should mark 

 12'' 0" 0' when the sun was highest. But the lengths of apparent solar 

 days, or the time elapsed between two successive apparent noons, are 

 not equal at different parts of the year, since the true sun does not 

 move in a plane perpendicular to the earth's rotation axis (the equa- 

 tor), but in the ecliptic, a plane greatly inclined to the equator, and 

 since the sun's motion in the ecliptic is not uniform. Hence arises an 

 inequality in these apparent solar days, and a capital advance was 

 made by the adoption of mean solar time, which is now universal. 

 Local mean noon is the time when an imaginary sun supposed to 

 move uniformly in the equator is on the meridian of any place, as 

 New York, and a mean solar day is the interval between two suc- 

 cessive mean noons. This is divided into twenty-four hours, and 

 these again into minutes and seconds, and the length of these imits is 

 practically invariable. 



The time of mean noon differs from the instant of apparent noon 

 no less than sixteen minutes at certain times in the year, being some- 



' Rigorously, the sun may not have its maximum altitude on the meridian, but its 

 maximum altitude can never differ from its meridian altitude by more than half a second 

 of arc. 



