Editing James Bradley (section)

==Works==
In 1722, Bradley measured the angular diameter of [[Venus]] with a large [[aerial telescope]] with an [[Objective (optics)|objective]] focal length of {{convert|212|ft|abbr=on}}.<ref name="eb" />

Bradley's discovery of the [[Aberration (astronomy)|aberration of light]] was made while attempting to detect stellar [[parallax]].<ref name="Hirshfeld" /> Bradley worked with [[Samuel Molyneux]] until Molyneux's death in 1728, trying to measure the parallax of [[Gamma Draconis]].

This stellar parallax ought to have shown up, if it existed at all, as a small annual cyclical motion of the apparent position of the star. However, while Bradley and Molyneux did not find the expected apparent motion due to parallax, they found instead a different and unexplained annual cyclical motion.  Shortly after Molyneux's death, Bradley realised that this was caused by what is now known as the [[aberration of light]].{{efn|[https://upload.wikimedia.org/wikipedia/commons/7/72/Bradley-New_Discovered_Motion_of_the_Fixed_Stars.pdf ''A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford, and F.R.S. to Dr. Edmond Halley Astronom. Reg. &c. Giving an Account of a New Discovered Motion of the Fix'd Stars.''] Philosophical Transactions 35 (406), no. 1727: 637–61.}}<ref name="Hirshfeld" /> The basis on which Bradley distinguished the annual motion actually observed from the expected motion due to parallax, was that its annual timetable was different.

Calculation showed that if there had been any appreciable motion due to parallax, then the star should have reached its most southerly apparent position in December, and its most northerly apparent position in June.  What Bradley found instead was an apparent motion that reached its most southerly point in March, and its most northerly point in September; and that could not be accounted for by parallax: the cause of a motion with the pattern actually seen was at first obscure.

A story has often been told, probably apocryphally, that the solution to the problem eventually occurred to Bradley while he was in a sailing boat on the [[River Thames]].  He noticed that when the boat turned about, a small flag at the top of the mast (a telltale) changed its direction, even though the wind had not changed; the only thing that had changed was the direction and speed of the boat. Bradley worked out the consequences of supposing that the direction and speed of the earth in its orbit, combined with a consistent speed of light from the star, might cause the apparent changes of stellar position that he observed.  He found that this fitted the observations well, and also gave an estimate for the speed of light, and showed that the stellar parallax, if any, with extremes in June and December, was far too small to measure at the precision available to Bradley.  (The smallness of any parallax, compared with expectations, also showed that the stars must be many times more distant from the Earth than anybody had previously believed.)

This discovery of what became known as the aberration of light was, for all realistic purposes, conclusive evidence for the movement of the Earth, and hence for the correctness of [[Aristarchus of Samos|Aristarchus]]', [[Nicolaus Copernicus|Copernicus]]', and [[Johannes Kepler|Kepler]]'s theories; it was announced to the Royal Society in January 1729.<ref>{{cite journal |last1=Bradley |first1=James |title=A Letter from the Reverend Mr. James Bradley Savilian Professor of Astronomy at Oxford, and F.R.S. to Dr. Edmond Halley Astronom. Reg. &c. Giving an Account of a New Discovered Motion of the Fix'd Stars |journal=Philosophical Transactions of the Royal Society of London |date=1728 |volume=35 |pages=637–661 |url=https://www.biodiversitylibrary.org/item/196465#page/305/mode/1up}}  On pp. 646–647 Bradley explains how the finite speed of light causes distant stars to appear to move along an elliptical orbit.  On p. 653 he presents his estimate of the speed of light.</ref>  The theory of the aberration also gave Bradley a means to improve on the accuracy of the previous estimate of the [[speed of light]], which had previously been estimated by the work of [[Ole Rømer]] and others.<ref name="Hirshfeld" /> Bradley's published value for the speed of light, expressed as a time of 8 minutes 12 seconds for light to travel the distance between the Earth and the Sun, was only about 1.3% too high, and was the first accurate determination of this fundamental physical constant.

The earliest observations upon which the discovery of the aberration was founded were made at [[Kew Palace#Fortreys and Capels|Molyneux's house on Kew Green]], and were continued at the house of Bradley's uncle James Pound in [[Wanstead]], Essex.  After publication of his work on the aberration, Bradley continued to observe, to develop and check his second major discovery, the [[astronomical nutation|nutation]] of the Earth's axis, but he did not announce this in print until 14 February 1748, when he had tested its reality by minute observations during an entire revolution (18.6 years) of the moon's nodes.<ref>{{cite journal |last1=Bradley |first1=James |title=A letter to the Right honourable George Earl of Macclesfield concerning an apparent motion observed in some of the fixed stars |journal=Philosophical Transactions of the Royal Society of London |date=1748 |volume=45 |issue=485 |pages=1–43 |url=https://www.biodiversitylibrary.org/item/206539#page/15/mode/1up}}</ref>

The publication of Bradley's observations was delayed by disputes about their ownership; but they were finally issued by the [[Clarendon Press]], Oxford, in two folio volumes (1798, 1805). The insight and industry of the German astronomer [[Friedrich Wilhelm Bessel]] were, however, needed for the development of their fundamental importance.<ref name="EB1911"/><ref>F. W. Bessel (1818): ''Fundamenta Astronomiae pro anno MDCCLV deducta ex observationibus viri incomparabilis James Bradley in specula astronomica Grenovicensi, per annos 1750–1762 institutis'', Königsberg.</ref>