General Methods of Work 15 



accordingly a losing rate is indicated by a positive sign and a gaining rate by a 

 negative sign. In the office computations for longitude, all of the timepieces 

 carried are used in the determination of the finally adopted correction of the 

 chronometer to Greenwich mean time. 



The observations for azimuth and time, whenever possible, consist of both 

 morning and afternoon alt-azimuth observations on the Sun at about the same 

 hour-angles, preferably not less than 2 hours from apparent noon. When the 

 readings on mark for vertical circle right and left have been taken, the usual order 

 of pointings on the Sun is as follows: beginning with vertical circle left, two suc- 

 cessive pointings on the Sun bringing two of its limbs tangent to the diaphragm cross- 

 lines in the quadrant, chosen so that the Sun apparently moves off one line and 

 across the other; next two successive pointings on the Sun in the opposite quadrant 

 with vertical circle reversed or right; for each pointing are recorded the chronometer 

 time, horizontal circle reading vernier A and vernier B, vertical circle reading 

 vernier A and vernier B, this completing one set. After any developed difference 

 in level has been corrected, the second set is made in the reverse order of position 

 and pointing to that of the first, making sights 5 and 6 same as 3 and 4, and 7 and 

 8 same as 1 and 2, concluding with mark readings, vertical circle left and right. 

 Under the proper headings as shown in the specimen observation on p. 32, there 

 are entered the precise mark used, position of the vertical circle, whether on left 

 hand (L) or on right hand (i?) side, when sighting on object from eyepiece end of 

 telescope, and the apparent limb of the Sun observed upon. 



The latitude is usually determined from circummeridian altitudes of the Sun, 

 beginning about 10 minutes before and continuing about 10 minutes after local 

 mean noon. The pointings are made on the upper and lower limbs of the Sun in 

 pairs, the position of vertical circle being shown in specimen on p. 31. 



Weather conditions, or other considerations, may not always permit strict 

 adherence to the above scheme of astronomical work, and the question as to the 

 desirability of spending additional time may, in consequence, arise; remaining at 

 one station for any length of time may occasion considerable additional expense 

 and subsequent delay. The relation of error in azimuth to the error in latitude is 

 expressed hy dA = cot/ sec (^ d<f>; it is thus seen that with nearly equal hour 

 angles, i. e., if for morning and afternoon observations the altitudes are nearly equal, 

 the resulting mean value of the azimuth is almost free from error due to the use of 

 an erroneous latitude as derived, for example, from a map. Hence it may not 

 always be worth while for an observer to remain at a station until he has secured 

 a regular set of latitude observations, especially as the latitude generally obtainable 

 from maps suffices for the prime purposes of the work. Sometimes it is possible 

 to obtain only a morning or an afternoon set of azimuth observations in addition 

 to the latitude observations; in this case also the observer is not required to wait 

 over for a second azimuth determination. Sometimes but one or two exmeridian 

 pointings on the Sun for latitude observations can be obtained; these will generally 

 furnish a sufficiently good determination if they are made within 20 minutes of 

 local apparent noon. 



