Observations pour déterminer la parallaxe annuelle de la 61e étoile du Cygne. - Extrait d'une Lettre de M. Bessel à M. de Humboldt. Note de M.Arago sur le même sujet. (Séance du Lundi 5 Novembre 1838). (+) Nouvelles observations sur la parallaxe annuelle de la 61e étoile du Cygne. - Lettre de M. Bessel à M. Arago. (Séance du Lundi 4 Mai 1840). (2 Papers).

Paris, Bachelier, 1838 a. 1840.

4to. No wrappers. In: "Comptes Rendus Hebdomadaires des Séances de L'Academie des Sciences", Tome 7, No 19 and Tome 10, No 17/18. With title-ages to vol. 7 and 10. Pp. (769-) 803 a. pp. (671-) 717. (Entire issues offered). Bessel's papers: pp. 785-793 a. pp. 703-710. Stamp on both titlepages. The second titlepage with a fes brownspots, otherwise clean and fine.

First appearance of a milestone paper in astronomy, giving the solution of the great problem of distances in the universe which had baffled astronomers ever since the time of Copernicus, announcing the FIRST SUCCESSFUL DISPLACEMENT OR PARALLAX OF A FIXED STAR and hence deducing the FIRST RELIABLE DISTANCE OF THE EARTH TO A FIXED STAR. The parallax observed corresponded to ab. 600.000 times that of the earth from the sun. On these grounds Bessel calculated the distance to about 11 light years, and this was confirmed by fresh investigations by Bessel in 1839-40 (the second paper offered). In 1842-43 it was also confirmed by C.A.F. Peters at Pulkowa. It is the first published instance of the fathom-line thrown into celestial space.

Bessel communicated his observations in Comptes Rendus, in a letter to Humboldt (the offered paper dated Nov. 5, 1838), in "Monthly Notices" in letter to J. Herschel, and in "Astronomische Nachrichten" Vol. 16, No 365-66 (pp. 65-96), 1838), where a more detail account was published.

Ther paper "Bestimmung der Entfernung des 61sten Stern des Schwans" in "Astronomische Nachrichten" is dated at the end: Altona 1838, Dec. 13.

Bessel's investigation was hailed by John Herschel when Bessel was awarded the R.A.S. gold medal "The greatest and most glorious triumph which practical astronomy has ever witnessed".

"For determining the parallax of 61 Cygni, Bessel selected two comparison stars of magnitude 9-10 at distances of roughly eight and twelve minutes of arc. 61 Cygni is a physical double star whose components differ in brightness by less than one magnitude. The distance of sixteen seconds of arc between the components favored the accuracy of the determination of the parallax because pointing could be carried out with two star images. After observing for eighteen months, by the fall of 1838 Bessel had enough measurements for the determination of a reliable parallax. He found that p = 0.314? with a mean error of ±:0.020?. This work was published in the Astronomische Nachrichten (1838), the first time the distance of a star became known. Bessel’s value for the parallax shows excellent agreement with the results obtained by extensive modern photographical parallax determinations,..." (DSB).

Parkinson "Breakthroughs" 1838 A. - Shapley & Howarth "A Source Book in Astronomy", pp. 216 ff.

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