Hooke, in 1674, published his observations of y Draconis, a star of the second magnitude which passes practically overhead in the latitude of London, and whose observations are therefore singularly free from the complex corrections due to astronomical refraction, and concluded that this star was 23" more northerly in July than in October.
They determined to reinvestigate the motion of y Draconis; the telescope, constructed by George Graham (1675-1751), a celebrated instrument-maker, was affixed to a vertical chimneystack, in such manner as to permit a small oscillation of the eyepiece, the amount of which, i.e.
The instrument was set up in November 1725, and observations on y Draconis were made on the 3rd, 5th, 11th, and 12th of December.
One hypothesis was: while y Draconis was stationary, the plumb-line, from which the angular measurements were made, varied; this would follow if the axis of the earth varied.
Nutation of the axis would determine a similar apparent motion for all stars: thus, all stars having the same polar distance as y Draconis should exhibit the same apparent motion after or before this star by a constant interval.