Bessel's Comet Candidate of November 9, 1808

24-year old Friedrich Wilhelm Bessel was looking for the comet observed in March 1808, that Olbers and he had calculated to have brightened again, to provide hope for a rediscovery in the evening sky.

The search was further motivated by a very clear sky on November 7 and the following days. On November 7, 1808, Bessel perceived two objects that appeared nebulous at first sight, one of which turned out to be a simple star in clearer air. These objects were observed in the same place in the following night, thus non-comets. On November 9, a fine nebulous patch was visible in very clear air; it was situated close to two stars in the Histoire céleste, the positions of which were: 

       a ... 7th mag    64d 51' 33".5   58d 58' 58".6
       b ... 9th mag    64  57   4 .7   58  53  46
Three other stars, which Bessel determined by own observation, were at 
       c .. 10th mag    65d  0' 26".4   58d 48'  4".4
       f .. 12th mag    65  10   3 .9   58  47  53 .7
       g .. 12th mag    65  14  12 .9   58  46  59 .2
With these stars, from 7:15 to 7:40 (p.m.) when it became cloudy again, the nebula formed the following configuration: In the reversing telescope, it was placed almost in the prolongation of the line from a to b, but slightly right of it, and its separation from b was estimated at 2/3 of a b ; a straight line through g f hit it in a distance of f > 2/3 g f ; a very fine starlet e, which was situated right of c and f and formed an equi-lateral triangle, can be used to determine the nebula more acurately, as it was situated on the straight line c e, little more than 2/3 c e from c. The position following from these estimates is = 65d 4' RA and 58d 50' Decl. On November 10 it was hazy; only on the 11th, at very serene sky, the nebula was no more visible in the place it had been on the 9th; instead, something nebulous occurred in 64d 21' [RA] and 58d 42' Decl., which however appeared more starlike on the 12th. Neither on the 12th nor on the 13th anything could be detected with certainty, which could have been taken for the comet.

Bessel was quite certain to have seen something on November 9 which was no more present later, but equally that it had not been the comet he originally had looked for, as this should have been visible in other evenings also. He had used Newtonian reflectors of 10.1" and 12" aperture for his observations.

Reference:

Hartmut Frommert
[SEDS]