Astronomical discovery . else to disturb the positionbut * parallax (the apparent shift due to theearths motion which it was desirable to find),this star ought to be a specially favourable objectfor the determination of parallax. Indeed it hadbeen announced many years before by Hooke thatits parallax had been found ; but his observationswere not altogether satisfactory, and it was witha view of either confirming them or seeing whatwas wrong with them that Molyneux and Bradleystarted their search. They set up a much more BRADLEYS DISCOVERIES 97 delicate piece of apparatus than Hooke had em-ploy
Astronomical discovery . else to disturb the positionbut * parallax (the apparent shift due to theearths motion which it was desirable to find),this star ought to be a specially favourable objectfor the determination of parallax. Indeed it hadbeen announced many years before by Hooke thatits parallax had been found ; but his observationswere not altogether satisfactory, and it was witha view of either confirming them or seeing whatwas wrong with them that Molyneux and Bradleystarted their search. They set up a much more BRADLEYS DISCOVERIES 97 delicate piece of apparatus than Hooke had em-ployed. It was a telescope 24 feet long pointed Thein- ^ -^ ^ 1-1-1 struraent. vertically upwards to the star, and firmly attachedto a large stack of brick chimneys within thehouse. The telescope was not absolutely fixed, forthe lower end could be moved by a screw so as tomake it point accurately to the star, and a plumb-line showed how far it was from the vertical whenso pointing. Hence if the star changed its posi- MARCH^. Fig. 2. tion, however slightly, the reading of this screwwould show the change. Now, before setting out Expectedon the observations, the observers knew what to ^^^^expect if the star had a real parallax; that is tosay, they knew that the star would seem to be farthest south in December, farthest north inJune, and at intermediate positions in March andSeptember; though they did not know hoio muchfarther south it would appear in December thanin June—this was exactly the point to be 98 ASTRONOMICAL DISCOVERY The reason of this will be clear from Fig. 2.[Remark, however, that this figure and the cor-responding figure 4 do not represent the case ofBradleys star, 7 Draconis : another star has beenchosen which simplifies the diagram, though theprinciple is essentially the same.] Let A B C Drepresent the earths orbit, the earth being atA in June, at B in September, and so on,and let K represent the position of the star onthe line D B. Then in March and Sept
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