The Solar Meridian 



Direct Solar Observation 



Inasmuch as the direct method of deriving a solar meridian is 

 widely exploited, we feel it a privilege to undertake a fresh review 

 of the argument. Unless the theodolite is equipped with a solar 

 attachment, this is the only reliable means available for establishing 

 a true meridian in the daytime with the single exception of the 

 Canadian method of observing polaris. 



Compared with the solar attachment it is contended* that the 

 theory is not more complex; that while the computations are longer 

 they are not beyond the ability of the modern surveyor; that no 

 other instrumental equipment is necessary than a good transit with 

 Accurate vertical circle and perhaps an eyepiece prism or diagonal 

 eyepiece with a sun filter of dense glass; that check observations can 

 be made in more rapid succession; that no preliminary preparation 

 or computations are necessary, and that by the system of reversals 

 required good results may be expected when the transit is out of 

 adjustment. 



On the other hand, the moment of observation is recorded more 

 precisely and the extensive computation entailed by check observa- 

 tions offers repeated opportunities for error in the calculations. 

 Observations should not be made when the sun is less than 15 

 above the horizon on account of uncertain refrartion, nor at any- 

 time between 11 a. m. and 1 p. m. when the movement of the sun 

 in altitude is insignificant compared to his movement in azimuth. 

 Double reversed observations do not compensate for errors in level- 

 ing the instrument. Any appreciable error in this respect (see I-b, 

 p. 63) \\i\\ cause discrepancies in the measured altitude and in the 

 final bearing. If there is an index error in the vertical circle, it 

 should be noted. (See VII, p. 71.) 



The difficulty of bisection with ordinary cross wires, due to the 

 size of the sun, requires attention and special expedients. The 

 diameter of the sun is 32', or thereabouts, in the vertical plane, but 

 his diameter in the horizontal plane varies as 32' -J- cos Alt. On this 

 account some operators take the average of several bisections. An 

 expert observer will be able to make a double set of observations 

 including eight pointings on the sun, four on the reference mark 

 and eight watch readings within 15 to 20 min. One set of observa- 

 tions will be taken v, ith the telescope erect and the second with it 

 inverted. This implies the necessity of having the transit equip- 

 ped with full vertical circle. The average of six readings, or even 

 four, ought to give reliable data for the sun's center, but one 

 should not be excused for thus attempting to correct tin 

 a poorly adjusted instrument.** 



* SffProf. W. H. Burger in Trans. III. Soc. Surv. & En., 1912. 

 * SffProf. C. E. Ro-ue - 488. 



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