Parallax : Bessel and Henderson

The following is taken from

J L E Dreyer, The decade 1830-1840, in History of the Royal Astronomical Society (Royal Astronomical Society, 1923), 50-81

and
R A Sampson, The decade 1840-1850, in History of the Royal Astronomical Society (Royal Astronomical Society, 1923), 82-109.

1. Henderson at the Cape.

A paper on refraction near the horizon was one of the results of Henderson's short stay at the Cape Observatory. With the mural circle he observed the apparent zenith distances of stars culminating within 5° of the horizon, on both sides of the zenith. The result was that the observations, except in the case of four or five stars, agreed better with the tables of Ivory than with those of Bessel.

A new value of the lunar parallax was another fruit of Henderson's Cape observations. He deduced it from observations of the moon's declination made with the mural circle at the Cape in 1832 and 1833, combined with corresponding observations made at Greenwich and Cambridge.

But valuable as these results of what might be called Henderson's expedition to the Cape undoubtedly are, they are thrown into the shade by his great achievement, the first reliable determination of the annual parallax of a fixed star. The astronomical world had grown rather tired of announcements of annual parallax found from meridian observations. Brinkley's parallaxes had been vigorously assailed by Pond; and though the question remained in doubt for some years, it was gradually recognised that they were imaginary. Henderson's paper was laid before the Society on 11 January 1839. By the long delay in reducing his observations of α Centauri he lost the priority of publication, as Bessel had announced the discovery of the parallax of 61 Cygni to the Society two months earlier.

2. Henderson appointed Astronomer Royal for Scotland.

In 1834 November, Baily, as President, announced that he had during the recess (probably early in August) received a letter from Lord Melbourne, First Lord of the Treasury, requesting that the Council would wait on him in order to recommend a proper person to fill the post of His Majesty's Astronomer at the Edinburgh Observatory, the administration of which had recently been taken over by the Government. As there was not time to call a meeting, Baily and four others had waited on Melbourne, and recommended Henderson; and this was approved by the Council. [Note. Thomas Carlyle was a candidate for the post and thought himself ill-used by his friend Jeffrey, then Lord Advocate, "who gave the office to a law-clerk." As a youth, Henderson had been a writer's clerk.] Henderson received the appointment and started work at once; his observations made up to the end of 1835 were sent to the Council in 1836 to be reported on, as to whether they ought to be printed ; and the same was done the following year, till the Home Office had got to understand that this precaution was unnecessary. The printing of observations seems at that time never to have been undertaken by any public body without the Society being consulted.

3. Parallax: Bessel and Henderson.

Parallax: Bessel and Henderson. - This careful balance [guarding against the undue influence of national feelings when awarding the Gold Medal] is well exemplified in the case of the Medal awarded to Bessel for the first unquestionable determination of a stellar parallax. This was in 1841. Astronomy had long been plagued with will-o'-the-wisp parallaxes. The annual variation that was looked for opened possibilities of confusion with seasonal changes which might be atmospheric or of various other kinds. The meridian methods employed were not well adapted either for absolute or the highest class of differential determination. Unless a clear, confirmed progression could be shown month by month through the cycle, which could arise from no other cause, suspense and even scepticism was the proper attitude. It has already been mentioned above that Henderson had returned from the Cape in 1833, bringing with him his observations for reduction. He aimed to be, and was, as thorough and careful of instrumental details as Bessel himself, and his discussion of the removal of errors from readings of the Cape mural circle was accepted as a model. In 1839 he produced his discussion of observations of α Centauri. The declination, subjected to every test that he could put, agreed with a parallax of about 1". Yet by common consent, perhaps not excluding Henderson's own, the matter was held as not proven, until Maclear, his successor, should produce a further series that would confirm it. The amount was felt to be large. We now know that the parallax is large, the modern accepted value is 0".76; it is the nearest lucid star yet found; but Struve had shown, twenty years earlier, that not one out of 27 circumpolar stars whose right ascensions he had examined possessed a parallax of half a second. The confirmation was forthcoming in 1842; but it was not reassuring that twenty other stars in Maclear's list showed an average prima facie parallax of 0".3. Not one of these has been confirmed. Henderson remarks : "In a conversation I had with M Bessel, "... whose friendship was his boast and delight, and whom he consciously took as his model in matters astronomical, ..."he expressed his wish that α Centauri were observed with a heliometer, or good equatorial, capable of precise micro-metrical measurement; he said he had doubts of the results derived from meridian instruments. He mentioned the case of Dr Brinkley's parallaxes, and stated that in his own observatory two excellent meridian circles, placed beside each other, gave at certain seasons places of the pole star that differed from each other; the reason of which disagreement he had not found out." On the other hand, Bessel's own heliometer measures of 61 Cygni left no shadow of doubt that the displacements observed were actually the proportionate projections of the earth's orbit and nothing else. All these points were surveyed critically in the most careful way by Main (Memoirs, 12). It was a just estimate of the actual position that awarded the Medal to Bessel in 1841. Henderson has sometimes been blamed for undue caution and delay. This seems a wrong view of the case; with the means at his disposal, caution and confirmation were an obligation. After his results were confirmed, the Council felt that he too should have recognition. But they missed the right opportunity for action. In 1843 the material was before them, and no name was proposed for the Medal. In 1844 November, Henderson's name was put forward, but in the same month he died. In the same month too, a painful, long and, as it proved, a fatal illness removed Bessel from the scene.

The figure of Bessel, loved and admired, has filled a prominent place in the development of astronomy; it will continue to do so; astronomy won him, with its peculiar appeal, in the first flush of his genius and strength, from his clerkship in a merchant's office. He established its foundation as much as it could be given to one man to do. It is surprising that a man with so great an impulse for thoroughness could bring so many works to definite conclusions. For example, he began his studies with the Königsberg heliometer by devoting a paper to the trigonometrical calculation of the field of its object glass. He was known in this country chiefly by his writings, but he visited it in 1842, when he passed a week, along with Jacobi, in Henderson's company at Edinburgh and in the Highlands, and stayed with Herschel, who learnt from him his intention of investigating the errors of Uranus on the hypothesis of an exterior planet.

Fame has given Bessel no more than he earned, but it has done less than justice to Henderson. There can be no thought of comparing the two men together; Henderson was avowedly a cultivator of the methods of others. "At the outset of his career he was led (probably by the commendation of them in our Memoirs) to study attentively the methods of the German astronomers, particularly those of Bessel and Struve, upon whose model he formed his practice, and from which he never departed." I would remark that as astronomy expands, the originator of methods, especially where they involve increase of labour, renders himself more and more ineffectual by his own advances, unless he finds unselfish, able, appreciative imitators to apply his methods far and wide. It needs those qualities, and imagination as well, to see that it is worth doing. Henderson never had a good instrument to work with. It was entirely due to his care that any result of value could be derived from observations with the Cape mural circle. After Henderson's time the circle was sent back to England in 1840 to be overhauled, and to Simms' and Airy's great astonishment it turned out that the steel collar was virtually loose upon the pivot; it had never been shrunk on, but was merely attached by soft solder. Yet Henderson undoubtedly exhibited the parallax of α Centauri in the measures of zenith distance derived with this instrument. All his other work was equally well judged. At the time of which we write he was living at Edinburgh, but he had formerly spent frequent periods in London, and so was well known to members of Council. Amiable and unobtrusive, he was very modest about his own merits. The biographical notice of his work in 1845 February is written from personal knowledge. "The character of Mr Henderson as an astronomer stands high, and his name will go down to posterity as an accurate observer, an industrious computer, a skilful manipulator, and an improver of methods in that department to which he devoted himself. ... Every observation is scrupulously discussed, ... his processes are fully explained, no labour is evaded, and no circumstance that can affect the accuracy of the final result is passed unnoticed. ... One of his most distinguishing qualities was sound judgment. He never attempted anything to which his powers were not fully equal; and, as a consequence of this, whatever he did he did well."

Last Updated March 2026