Editing Aberration (astronomy) (section)

===Early hypotheses===
[[File:Hypothetical movement of γ Draconis caused by parallax.jpg|thumb|Hypothetical observation of γ Draconis if its movement was caused by parallax.]]

[[File:Hypothetical movement of γ Draconis and 35 Camelopardalis caused by nutation.jpg|thumb|Hypothetical observation of γ Draconis and 35 Camelopardalis if their movements were caused by nutation.]]

Bradley and Molyneux discussed several hypotheses in the hope of finding the solution. Since the apparent motion was evidently caused neither by parallax nor observational errors, Bradley first hypothesized that it could be due to oscillations in the orientation of the Earth's axis relative to the celestial sphere&nbsp;– a phenomenon known as [[astronomical nutation|nutation]]. 35 Camelopardalis was seen to possess an apparent motion which could be consistent with nutation, but since its declination varied only one half as much as that of γ Draconis, it was obvious that nutation did not supply the answer{{sfnp|Eppenstein|1911|p=55}} (however, Bradley later went on to discover that the Earth does indeed nutate).<ref name=berry/> He also investigated the possibility that the motion was due to an irregular distribution of the [[Earth's atmosphere]], thus involving abnormal variations in the refractive index, but again obtained negative results.{{sfnp|Eppenstein|1911|p=55}}

On August 19, 1727, Bradley embarked upon a further series of observations using a telescope of his own erected at the Rectory, [[Wanstead]]. This instrument had the advantage of a larger field of view and he was able to obtain precise positions of a large number of stars over the course of about twenty years. During his first two years at Wanstead, he established the existence of the phenomenon of aberration beyond all doubt, and this also enabled him to formulate a set of rules that would allow the calculation of the effect on any given star at a specified date.