Oil Ihc Extra- 31 eridian Determination of Time. 211 



OX THE EXTRA-JIEJilDIAX nETEB3IINATI0X OF 



TIME BY JIEAXS OF A POUT ABLE TBAN81T 



IX STB U 31 EXT. 



By Okmoxd Stone, A. M.. Astronomer of the Cincinnati Observatoiy. 



It is a well-known fact that in the determination of time by means 

 of a portable transit, the principal source of error is the instability of 

 the instrument. It is important, therefore, that the observations be 

 so aiTanged that the instrumental errors may be considered constant 

 for as short an interval as possible. On this account it is usual to de- 

 termine the level error independently for each star. For the determi- 

 nation of the azimuth, transits of at least two stars must be observed 

 in the same position of the instrument ; for the determination of the 

 collimation, observations must be made in reversed positions of the 

 horizontal axis. The azimuth may be eliminated b}' observing in the 

 zenith, or by observing two stars whose declinations are such that 



tan o — tan (p = tan cp — tan 8, 

 where o is the latitude of the place. Although these conditions can 

 not in general be rigidW fulfilled, it has been for some time the custom 

 at this Observatory to approximatel}' accomplish the object desired, 

 by observing sets of four stars situated within 20° alternateh^ north 

 and south of the zenith, and, as nearh' as may be convenient, at equal 

 distances from it. 



One advantage of this method is that pole stars are avoided. When 

 observing in the field it is somewhat inconvenient to adjust the instru- 

 ment exactly in the meridian, and when this is accomplished, the ob- 

 server can not alwa3'S afford to wait for the passage of a circumpolar 

 star. Various methods have therefore been developed for the reduc- 

 tion of observations made in the vertical of the pole star, instead ot 

 the meridian. All of these methods are somewhat complicated, and 

 although earnestly recommended by some of the most competent 

 judges, especiall}' within the past few years by Professor Doellen, the 

 distinguished senior astronomer at the Poulkova Observatory, the}^ 

 have not come into general use, except perhaps in Russia. Observations 

 of pole stars are very convenient, and in fact necessar\-, for the deter- 

 mination of fundamental right ascensions with large fixed instruments, 

 but for the determination of time possess but little advantage, except 

 for purposes of orientation. 



