5 2o THE POPULAR SCIENCE MONTHLY. 



however, the recognized day in most countries till comparatively re- 

 cent times. In France, when in 1816 the change was made to our 

 present system, there were fears of a disturbance among working-peo- 

 ple, lest the abolition of the sun-day should somehow increase their 

 hours of labor. It met the approval of the watch-makers, however, 

 whose customers had hitherto complained that their watches would 

 not keep pace with the sun, not knowing that this would be impos- 

 sible for a good watch. 



The time of the rotation of the earth on its axis can not be meas- 

 ured directly from the sun, for the reason that the earth is moving 

 around it. We must have some external point, fixed with reference 

 to the earth, by which to measure it. The stars afford such points. 

 By noticing the time between two successive passages of a star over 

 our meridian (our meridian being, as is well known, the semicircle in 

 the sky passing from the north to the south point of the horizon di- 

 rectly overhead), we would obtain the exact time of the earth's com- 

 pleting one spin on its axis. This time, which is about four minutes 

 less than our ordinary day, is called in astronomical parlance a side- 

 real day, and, divided in the ordinary manner into hours, minutes, and 

 seconds, is known as sidereal time. It has no direct relations to ordi- 

 nary life. 



Through all the time that the earth is making one turn on its axis 

 it is advancing around the sun in the same direction. So it takes this 

 extra four minutes to bring the same meridian under the sun again, 

 after making a complete revolution. Hence we have our solar day. 

 Again, since the forward motion of the earth is not uniform, as well 

 as for another cause, which is too intricate to mention here, the solar 

 days are not, as we have said above, of equal length. So the device is 

 adopted of ascertaining their average through the year and calling it 

 the mean solar day. This, subdivided into hours, minutes, and sec- 

 onds, is mean time the clock-time of ordinary life. 



If, therefore, it is desired to find correct time from a sun-dial, or 

 by any method depending on the sun, the correction from apparent to 

 mean time must be made. At four instants during the year this cor- 

 rection is zero. At other times a quantity, amounting at its greatest to 

 about sixteen minutes, must be added to or subtracted from sun-time. 

 For several days in the early part of November the sun is on the 

 meridian more than a quarter of an hour before twelve o'clock. Our 

 present system is not exact sun-time, but sun-time so modified as to be 

 adapted to the current wants of our existence. It is uniform, because 

 it is based on the time of revolution of the earth on its axis, which has 

 not varied, if at all, more than one sixtieth of a second in the past 

 twenty-five hundred years. But the common day is not the exact 

 time of the earth's revolution, nor is the common year the exact time 

 of its motion around the sun. 



The tendency of civilization seems to be to depart from these strict 



