XXVI INTRODUCTION. 



Table 25. Local mean time at apparent noon. 



This table gives the local mean time ^ that should be shown by a clock 

 when the center of the sun crosses the meridian, on the 1st, 8th, i6th, and 

 24th days of each month. The table is useful in correcting a clock by means 

 of a sundial or noon mark. 

 Example : 



To find the correct local mean time when the sun crosses the meridian 



on December 15, 1891. 

 The table gives for December 16, 11'^ 56'". By interpolating, it is seen 

 that the change to December 15 would be only one-half minute; 

 the correct clock time is therefore 4 minutes before 12 o'clock 

 noon. 



Table 26. Sidereal time into mean solar time. 

 Table 27. Mean solar time into sidereal time. 



According to Newcomb, the length of the tropical year is 365.24220 

 mean solar days,'- whence 



365.24220 solar days = 366.24220 sidereal days. 

 Any interval of mean time may therefore be changed into sidereal time 



by Increasing it by its —^ — ^^ part, and any interval of sidereal time may 



be changed into mean time by diminishing it by its —77 part. 



^ .7 & .r 366.24220 ^ 



Table 26 gives the quantities to be subtracted from the hours, minutes 

 and seconds of a sidereal interval to obtain the corresponding mean time 

 interval, and Table 27 gives the quantities to be added to the hours, min- 

 utes and seconds of a mean time interval to obtain the corresponding side- 

 real interval. The correction for seconds is sensibly the same for either a 

 sidereal or a mean time interval and is therefore given but once, thus form- 

 ing a part of each table. 

 Examples: 



Change 14'^ 25"^ 36^2 sidereal time into mean solar time. 



Given sidereal time 



Correction for 14^ 



25" 

 36!2 



Corresponding mean time = 14 23 14.4 



1 Derived from the equation of time for Washington apparent noon for the year 

 1899. See the American Ephemeris and Nautical Almanac, 1899, pages 377-84. 



2 The length of the tropical year is not absolutely constant. The value here given is 

 for the year 1900. Its decrease in 100 years is about 0.5s. (See the American Ephemeris 

 and Nautical Almanac 1918, page xvi.) 



