The Bessel Legacy

In his epoch-making parallax measurement of 61 Cygni in 1838, F.W. Bessel had to make assumptions about the parallaxes and proper motions of the two comparison stars to which he referred his local sights. None of these stars was in the Hipparcos Catalogue and up to Gaia DR2, no check of these assumptions could be done with certainty.

Thanks to the Gaia DR2, it is possible to identify these stars and -- with the best orbit of 61 Cygni -- reconstruct Bessel's observations to confirm the soundness of the assumptions and estimate the single observation accuracy of his observations with the Heliometer. This is compared to the quoted accuracy and to the large scatter seen in the numerous other measurements of the parallax of 61 Cygni published until ~1900.

Presentation delivered at the 53rd ESLAB symposium held at ESTEC, The Netherlands, 8-12 April 2019.