thi> watch error in local mean lime is then found by taking the difference between the 

 watch reading at the epoch of the observation and the reduced local mean time of 

 observation. 



Local mean time of upper culmination of Polaris. The Greenwich mean time of 

 upper culmination of Polaris is to be taken from the Ephemeris for the date of obser- 

 vation; the amount to be subtracted from the Greenwich mean time of upper culmina- 

 tion of Polaris to obtain the local mean time of upper culmination of Polaris, in which 

 the argument is the longitude west from Greenwich, is obtained from the table of 

 sidereal conversions without computation. 



Local mean time of elongation of Polaris. The mean time of elongation of Polaris, 

 Greenwich meridian, latitude 40, is to be taken from the Ephemeris for the date of 

 observation; the amount to be subtracted from the mean time of elongation of Polaris, 

 Greenwich meridian, latitude 40, to obtain the mean time of elongation of Polaris, 

 local meridian, latitude 40, in which the argument is the longitude west from Green- 

 wich, is obtained from the table of sidereal conversions without computation. The 

 amount to apply to the local mean time of elongation of Polaris latitude 40 to obtain 

 the local mean time of elongation of Polaris latitude of observation is tabulated in 

 the Ephemeris in connection with the table of azimuths of Polaris at elongation. 



Conversion of a mean time interval into a sidereal time interval, or vice versa. fhe 

 amount to apply to one time interval to obtain the other time interval is obtained 

 from the table of sidereal conversions without computation. 



Hour angles of Polaris. A mean time hour angle of Polaris west of the meridian is 

 the mean time interval from the local mean time of upper culmination of Polaris to 

 the local mean time of observation of Polaris; a mean time hour angle of Polaris east 

 of the meridian is the mean time interval from the local mean time of observation of 

 Polaris to the local mean time of upper culmination of Polaris. 



Mean time hour angle of Polaris at elongation. " t "= the sidereal hour angle in an- 

 gular measure, this converted into time measure, and this in turn converted from a 

 sidereal time interval into a mean time interval gives the mean time hour angle of 

 Polaris at elongation: 



Cos<=cotan<J tan 4> 



Altitude observation of the sun for apparent time." t "=hour angle from apparent 

 noon in angular measure; reverse the signs of "d" for south declinations: 



Tan 1 t-*m (C+< *~ 3) Si " * <*- 

 - 



Meridian observation of the sun for apparent noon. With the telescope in the merid- 

 ian elevated to the sun's alii tude. tho watch times of transit of the sun's west and east 

 limbs are noted, the mean of which is the watch time of apparent noon; if the obser- 

 vation fails for either limb the reduction to the sun's center is accomplished by adding 

 or subtracting OK seconds; a refinement in the amount of this time is had by referring 

 to the Ephemeris for the time of the sun's semidiameter passing the meridian for the 

 date of observation; the setting for the approximate altitude of the sun's center is: 



LATITUDE. 



Meridian altitude observation of the sun for latitude. Reverse the sign of "*" for 

 south declinations: 



This observation is conveniently combined with the meridian observation of the 

 sun for time, observing simultaneously the sun's lower and west limbs, recording 

 the watch time and the vertical angle and reversing in tho interval of about 2} min- 



222 



