The zenith star that would not sit

At Molyneux’s house on Kew Green they mount a zenith sector on Gamma Draconis. If the star is close, it should nod with the Earth’s orbit: parallax. In December 1725 the star begins to drift. The drift continues at Wanstead, at the house of Bradley’s uncle James Pound. It is too big for the parallax they hoped for, and it does not keep the season of a parallax.

Bradley later sees that the Earth’s motion through the light is enough. The star’s apparent place is tipped by the speed of the observer. Aberration. A detection of the Earth’s motion, and of the finite speed of light, dressed as a failed distance.

The constant of aberration is about 20.5 arcseconds. Parallax for this star is far smaller. Bessel’s 0.314″ will be a different angle, a different star, a century later. Bradley found the motion of the Earth in the star’s apparent walk. He did not yet have 61 Cygni.