Bessel and the Royal Astronomical Society

In his article The decade 1840-1850, in J L E Dreyer and H H Turner (eds.), History of the Royal Astronomical Society (London, 1923), 82-109], R A Sampson gives an account of the achievements of Wilhelm Bessel who won the Gold Medal of the Royal Astronomical Society in 1829 and 1841. He also tells us that Bessel came to Scotland and visited Edinburgh and the Highlands of Scotland:-

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 foundations 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.

There are two obvious questions arising from this quotation. Did Bessel investigate the errors of Uranus on the hypothesis of an exterior planet? Who was the Henderson that Bessel went to visit in Edinburgh?

The answer to the first is that Bessel did start working on the problem of finding a planet beyond Uranus. He employed a young astronomer by the name of Flemming to assist him in his investigation and wrote to Herschel:-

Let me tell you that Uranus is not forgotten.

However Bessel's health began to deteriorate due to cancer and he was unable to continue with his work.

As to the second question, Henderson was the Astronomer Royal for Scotland. He was an extremely accurate observer and returned from the Cape in 1833 having made observations there for two years. In fact he took a long time to publish his results, and when he sent a paper to the Royal Astronomical Society on 11 January 1839 showing that Alpha Centauri showed a parallax of about 1" he had lost out to Bessel who had sent his proof of the parallax of 61 Cygni to the Society two months earlier. In fact Henderson was the first to correctly measure a stellar parallax but Bessel published first. It was the parallax of Alpha Centauri that Bessel wanted to discuss with Henderson when he visited Edinburgh in 1842. Henderson reported:-

In a conversation I had with M Bessel, he expressed his wish that Alpha Centauri were observed with a heliometer, or good equatorial, capable of precise micrometrical 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.

Of course another reason that Henderson's results were doubted was that he had found a surprisingly large parallax. this was, of course, because he had measured the distance to the nearest star to the sun, as was later confirmed.


JOC/EFR March 2006

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