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Bessel, Friedrich Wilhelm

parallax sirius measurement functions

(1784–1846) German astronomer and mathematician: made first measurement of a star’s distance by parallax; detected that Sirius had a companion; introduced Bessel functions.

As a young trainee accountant in Bremen, Bessel prepared for travel by studying navigation and then astronomy. This in turn took him, aged 26, to be director of the new Königsberg Observatory.

Much of Bessel’s work deals with the analysis of perturbations in planetary and stellar motions. For this purpose he developed the mathematical functions that now bear his name, publishing his results in 1824 in a paper on planetary perturbations. Bessel functions have subsequently proved to have wide application in other areas of physics. In 1838 he was the first to announce the measurement of a star’s distance by measurement of its parallax. Stellar parallax is the displacement the nearer stars should show (relative to distant ones) over time, because they are viewed at varying angles as the Earth moves across its orbit. Such parallax had not been observed previously; suggested that this was because all stars are so distant that the parallax is immeasurably small. Bessel was able to measure the parallax of the binary star 61 Cygni as 0.3” of arc and thus found its distance to be 10.3 light years (within 10% of the present value). Around the same time he observed a small wave-like motion of Sirius and suggested that it was the result of the gravitational influence of an unseen orbiting companion; the faint companion Sirius B was subsequently detected by Alvan G Clark (1832–97), a telescope lens maker, in 1862. Bessel also succeeded in computing the mass of the planet Jupiter by analysing the orbits of its major satellites and showed its overall density to be only 1.35. He suggested that irregularities in the orbit of Uranus were caused by the presence of an unknown planet, but died a few months before the discovery of Neptune.

Besseler, Heinrich [next] [back] Bessaraboff, Nicholas (actually, Nikolai)

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