James Bradley and the eighteenth Century 'Gap' in attempts to measure annual stellar parallax

Abstract

When, in 1728, James Bradley wrote to Edmund Halley of his ‘new discovered motion of the fixed stars’ (1) Bradley pointed out an important implication of his work for the problem of the detection of the annual parallax of the stars. In conclusion he wrote thus (2): I believe I may venture to say, that in either of the two stars last mentioned [the annual parallax] does not amount to 2". I am of opinion, that if it were 1". I should have perceived it in the great number of observations that I made, especially upon y Draconis; which agreeing with the hypothesis . . . nearly as well when the sun was in conjunction with, as in opposition to, this star, it seems very probable, that the parallax of it is not so great as one single second. This statement has been cited by historians as a decisive turning point in the history of attempts to measure parallax: a turning point in the wrong direction, however, as after this, it is claimed, astronomers no longer sought parallax measurements, believing such small angles to be beyond the limits of even the most sensitive instruments. According to M. A. Hoskin (3): Not surprisingly, Bradley’s revelation of the most incredible delicacy of the required measurements . . . and the apparent near impossibility of maintaining such accuracy over an annual cycle, resulted in a failure of nerve among those few astonomers who possessed instruments capable of precision measurements. And the Canadian astronomer J. D. Fernie suggests that (4): The great accuracy of Bradley’s observations and their failure to detect any star’s parallax seem to have put something of a damper on further attempts at direct absolute measurement for the remainder of the eighteenth century.