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INDIANA UNIVERSITY STUDIES 



To facilitate the choice of stars in making out an observing 

 hst, Hinks of Cambridge (Eng.) computed and constructed a chart 

 which is highly serviceable. It shows at once the two days of the 

 year when the parallactic displacement is greatest; the mean time 

 on those days when it is best suited for observing; and the hour 

 angle and zenith distance at which it is to be observed. This chart 

 may be found in The Monthly Notices of the Royal Astronomical 

 Society, Vol. 58, p. 142. Since the latitude enters into the compu- 

 tation, it is good for only one latitude and would have to be recom- 

 puted for any other latitude. 



In Sir David Gill's work on stellar parallax, pubhshed in the 

 Annals of the Royal Observatory, Cape of Good Hope, Vol. VIII, Pt. 

 II, is a table giving the longitude of the sun at the epoch of a star's 

 parallax maximum. This table is computed for every ten degrees 

 in declination, north and south, and every hour of right ascension. 

 Knowing the right ascension and dechnation of the star, it is a very 

 easy matter to turn to the Nautical Almanac and note the time cor- 

 responding to the longitude as taken from the table. 



The direction of the major axis of the parallactic eUipse was now 

 determined. For this purpose, Table I in Sir David Gill's work 

 referred to above, was used. This table gives the position angle of 

 the major axis of a star's parallactic ellipse for every ten degrees of 

 declination and each hour of right ascension. 



Method of Observing. The method used was that of deter- 

 mining the relative parallax; in fact no one attempts absolute 

 parallax now. One faint star was selected in the same field with 

 the star whose parallax was to be determined. The distance be- 

 tween the two stars was measured each night. The observations 

 covered the month preceding and the month following the time of 

 maximum displacement. Then six months later, at next maximum, 

 a similar set of measures was taken. Thus the observing of each 

 star extended through something like four months. Cloudy weather, 

 however, so interfered with observing, that this rule could not be 

 closely followed. In a few instances, no measures were secured 

 until after the time of maximum displacement. The position angle 

 was also observed often enough to get an approximate value for it. 

 It must be supposed that the faint star is so remote as to have little 

 or no parallax, and that if a displacement is found it is practically 

 all due to the brighter star. 



Before beginning work each night the eye-piece and then the 

 telescope were carefully focused. The dome was opened some- 

 time previous to the observing so that the room might be well 



