44 



Circum-Elongation observations for Azimuth. 

 By R. W. Chapman, M.A., B.C.E. 



[Read April 8, 1915.] 



On account of its convenience and comparative simplicity, 

 the observation of a circumpolar star at elongation is, amongst 

 surveyors, the favourite star observation for the determina- 

 tion of a true azimuth. The great disadvantage of the method 

 is that only one observation can be made with the star actually 

 at elongation, and there is thus no opportunity to eliminate 

 instrumental errors in the same way as may be done, when a 

 series of observations of the same star are made, by taking 

 half the readings with the instrument reversed. As a rule 

 the motion of the star in azimuth is so slow, when near to 

 elongation, that with an ordinary transit theodolite two 

 observations can be made and treated as though the star were 

 actually at elongation without introducing an error sufficient 

 to be measured by the instrument. But a much higher degree 

 of accuracy is possible with the method if a series of half a 

 dozen observations are made on each side of elongation, and 

 the object of the present paper is to discuss the convenient 

 reduction of such a series of observations. For the reduction 

 of a similar set of observations made upon a close circumpolar 

 star there is a well-known method that is particularly applic- 

 able to the Pole star of the Northern Hemisphere. Unfor- 

 tunately in the Southern Hemisphere our close circumpolar 

 stars are very faint and not easy to work with, a Octantis has a 

 polar distance between 46' and 47', but its magnitude is 5^^, so 

 that it is not readily picked out by the surveyor. The bright 

 southern stars that are most convenient for the determination 

 have commonly a polar distance of about 30°, and to these 

 the formula for close circumpolar stars cannot always be 

 applied without introducing appreciable error. 



Two methods are possible for a series of observations 

 made before and after elongation. We may read the verniers 

 of the horizontal circle and note the time at each observation, 

 or we may read the horizontal circle and also the altitude of 

 the star at each observation. The former method is prefer- 

 able, provided that the surveyor has the correct local time, as 

 errors due to a defective knowledge of atmospheric refraction 

 are not then introduced. The latter method, however, 

 involves no knowledge of the time, and is much more con- 

 venient when the observations have to be carried out single- 

 handed. In both cases the azimuth of the star at each 



