In 1812 Bessel measured with it the angle between the components of the double star 61 Cygni and observed the great comet of 1811.
X Cygni is another star of this class, remarkable for its range of magnitude.
He chose for his purpose the binary star 61 Cygni, which was the star with the most rapid apparent motion then known and therefore likely to be fairly near us, although only of the sixth magnitude.
3 i for 61 Cygni was the first authentic result of the kind published.
Bessel announced, in December 1838, the perspective yearly shifting of 61 Cygni in an ellipse with a mean radius of about one-third of a second.